| Message ID | 20250327092922.536-1-link@vivo.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | Deep talk about folio vmap | expand |
On Thu, Mar 27, 2025 at 05:28:27PM +0800, Huan Yang wrote: > Bingbu reported an issue in [1] that udmabuf vmap failed and in [2], we > discussed the scenario of folio vmap due to the misuse of vmap_pfn > in udmabuf. > > We reached the conclusion that vmap_pfn prohibits the use of page-based > PFNs: > Christoph Hellwig : 'No, vmap_pfn is entirely for memory not backed by > pages or folios, i.e. PCIe BARs and similar memory. This must not be > mixed with proper folio backed memory.' > > But udmabuf still need consider HVO based folio's vmap, and need fix > vmap issue. This RFC code want to show the two point that I mentioned > in [2], and more deep talk it: > > Point1. simple copy vmap_pfn code, don't bother common vmap_pfn, use by > itself and remove pfn_valid check. > > Point2. implement folio array based vmap(vmap_folios), which can given a > range of each folio(offset, nr_pages), so can suit HVO folio's vmap. > > Patch 1-2 implement point1, and add a test simple set in udmabuf driver. > Patch 3-5 implement point2, also can test it. > > Kasireddy also show that 'another option is to just limit udmabuf's vmap() > to only shmem folios'(This I guess folio_test_hugetlb_vmemmap_optimized > can help.) > > But I prefer point2 to solution this issue, and IMO, folio based vmap still > need. > > Compare to page based vmap(or pfn based), we need split each large folio > into single page struct, this need more large array struct and more longer > iter. If each tail page struct not exist(like HVO), can only use pfn vmap, > but there are no common api to do this. > > In [2], we talked that udmabuf can use hugetlb as the memory > provider, and can give a range use. So if HVO used in hugetlb, each folio's > tail page may freed, so we can't use page based vmap, only can use pfn > based, which show in point1. > > Further more, Folio based vmap only need record each folio(and offset, > nr_pages if range need). For 20MB vmap, page based need 5120 pages(40KB), > 2MB folios only need 10 folio(80Byte). > > Matthew show that Vishal also offered a folio based vmap - vmap_file[3]. > This RFC patch want a range based folio, not only a full folio's map(like > file's folio), to resolve some problem like HVO's range folio vmap. Hmmm, I should've been more communicative, sorry about that. V1 was poorly implemented, and I've had a V2 sitting around that does Exactly what you want. I'll send V2 to the mailing list and you can take a look at it; preferrably you integrate that into this patchset instead (it would make both the udma and vmalloc code much neater). > Please give me more suggestion. > > Test case: > //enable/disable HVO > 1. echo [1|0] > /proc/sys/vm/hugetlb_optimize_vmemmap > //prepare HUGETLB > 2. echo 10 > /sys/kernel/mm/hugepages/hugepages-2048kB/nr_hugepages > 3. ./udmabuf_vmap > 4. check output, and dmesg if any warn. > > [1] https://lore.kernel.org/all/9172a601-c360-0d5b-ba1b-33deba430455@linux.intel.com/ > [2] https://lore.kernel.org/lkml/20250312061513.1126496-1-link@vivo.com/ > [3] https://lore.kernel.org/linux-mm/20250131001806.92349-1-vishal.moola@gmail.com/ > > Huan Yang (6): > udmabuf: try fix udmabuf vmap > udmabuf: try udmabuf vmap test > mm/vmalloc: try add vmap folios range > udmabuf: use vmap_range_folios > udmabuf: vmap test suit for pages and pfns compare > udmabuf: remove no need code > > drivers/dma-buf/udmabuf.c | 29 +++++++++----------- > include/linux/vmalloc.h | 57 +++++++++++++++++++++++++++++++++++++++ > mm/vmalloc.c | 47 ++++++++++++++++++++++++++++++++ > 3 files changed, 117 insertions(+), 16 deletions(-) > > -- > 2.48.1 >
After the btrfs compressed bio discussion I think the hugetlb changes that skip the tail pages are fundamentally unsafe in the current kernel. That is because the bio_vec representation assumes tail pages do exist, so as soon as you are doing direct I/O that generates a bvec starting beyond the present head page things will blow up. Other users of bio_vecs might do the same, but the way the block bio_vecs are generated are very suspect to that. So we'll first need to sort that out and a few other things before we can even think of enabling such a feature.
> On Apr 4, 2025, at 17:01, Christoph Hellwig <hch@lst.de> wrote: > > After the btrfs compressed bio discussion I think the hugetlb changes that > skip the tail pages are fundamentally unsafe in the current kernel. > > That is because the bio_vec representation assumes tail pages do exist, so > as soon as you are doing direct I/O that generates a bvec starting beyond > the present head page things will blow up. Other users of bio_vecs might > do the same, but the way the block bio_vecs are generated are very suspect > to that. So we'll first need to sort that out and a few other things > before we can even think of enabling such a feature. > I would like to express my gratitude to Christoph for including me in the thread. I have carefully read the cover letter in [1], which indicates that an issue has arisen due to the improper use of `vmap_pfn()`. I'm wondering if we could consider using `vmap()` instead. In the HVO scenario, the tail struct pages do **exist**, but they are read-only. I've examined the code of `vmap()`, and it appears that it only reads the struct page. Therefore, it seems feasible for us to use `vmap()` (I am not a expert in udmabuf.). Right? [1] https://lore.kernel.org/linux-mm/20250327092922.536-1-link@vivo.com/T/#m055b34978cf882fd44d2d08d929b50292d8502b4 Thanks, Muchun.
> On Apr 4, 2025, at 17:38, Muchun Song <muchun.song@linux.dev> wrote: > > > >> On Apr 4, 2025, at 17:01, Christoph Hellwig <hch@lst.de> wrote: >> >> After the btrfs compressed bio discussion I think the hugetlb changes that >> skip the tail pages are fundamentally unsafe in the current kernel. >> >> That is because the bio_vec representation assumes tail pages do exist, so >> as soon as you are doing direct I/O that generates a bvec starting beyond >> the present head page things will blow up. Other users of bio_vecs might >> do the same, but the way the block bio_vecs are generated are very suspect >> to that. So we'll first need to sort that out and a few other things >> before we can even think of enabling such a feature. >> > > I would like to express my gratitude to Christoph for including me in the > thread. I have carefully read the cover letter in [1], which indicates > that an issue has arisen due to the improper use of `vmap_pfn()`. I'm > wondering if we could consider using `vmap()` instead. In the HVO scenario, > the tail struct pages do **exist**, but they are read-only. I've examined > the code of `vmap()`, and it appears that it only reads the struct page. > Therefore, it seems feasible for us to use `vmap()` (I am not a expert in > udmabuf.). Right? I believe my stance is correct. I've also reviewed another thread in [2]. Allow me to clarify and correct the viewpoints you presented. You stated: " So by HVO, it also not backed by pages, only contains folio head, each tail pfn's page struct go away. " This statement is entirely inaccurate. The tail pages do not cease to exist; rather, they are read-only. For your specific use-case, please use `vmap()` to resolve the issue at hand. If you wish to gain a comprehensive understanding of the fundamentals of HVO, I kindly suggest a thorough review of the document in [3]. [2] https://lore.kernel.org/lkml/5229b24f-1984-4225-ae03-8b952de56e3b@vivo.com/#t [3] Documentation/mm/vmemmap_dedup.rst > > [1] https://lore.kernel.org/linux-mm/20250327092922.536-1-link@vivo.com/T/#m055b34978cf882fd44d2d08d929b50292d8502b4 > > Thanks, > Muchun. >
在 2025/4/4 18:07, Muchun Song 写道: > >> On Apr 4, 2025, at 17:38, Muchun Song <muchun.song@linux.dev> wrote: >> >> >> >>> On Apr 4, 2025, at 17:01, Christoph Hellwig <hch@lst.de> wrote: >>> >>> After the btrfs compressed bio discussion I think the hugetlb changes that >>> skip the tail pages are fundamentally unsafe in the current kernel. >>> >>> That is because the bio_vec representation assumes tail pages do exist, so >>> as soon as you are doing direct I/O that generates a bvec starting beyond >>> the present head page things will blow up. Other users of bio_vecs might >>> do the same, but the way the block bio_vecs are generated are very suspect >>> to that. So we'll first need to sort that out and a few other things >>> before we can even think of enabling such a feature. >>> >> I would like to express my gratitude to Christoph for including me in the >> thread. I have carefully read the cover letter in [1], which indicates >> that an issue has arisen due to the improper use of `vmap_pfn()`. I'm >> wondering if we could consider using `vmap()` instead. In the HVO scenario, >> the tail struct pages do **exist**, but they are read-only. I've examined >> the code of `vmap()`, and it appears that it only reads the struct page. >> Therefore, it seems feasible for us to use `vmap()` (I am not a expert in >> udmabuf.). Right? > I believe my stance is correct. I've also reviewed another thread in [2]. > Allow me to clarify and correct the viewpoints you presented. You stated: > " > So by HVO, it also not backed by pages, only contains folio head, each > tail pfn's page struct go away. > " > This statement is entirely inaccurate. The tail pages do not cease to exist; > rather, they are read-only. For your specific use-case, please use `vmap()` > to resolve the issue at hand. If you wish to gain a comprehensive understanding I see the document give a simple graph to point: +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ | | | 0 | -------------> | 0 | | | +-----------+ +-----------+ | | | 1 | -------------> | 1 | | | +-----------+ +-----------+ | | | 2 | ----------------^ ^ ^ ^ ^ ^ | | +-----------+ | | | | | | | | 3 | ------------------+ | | | | | | +-----------+ | | | | | | | 4 | --------------------+ | | | | PMD | +-----------+ | | | | level | | 5 | ----------------------+ | | | mapping | +-----------+ | | | | | 6 | ------------------------+ | | | +-----------+ | | | | 7 | --------------------------+ | | +-----------+ | | | | | | +-----------+ If I understand correct, each 2-7 tail's page struct is freed, so if I just need map page 2-7, can we use vmap do something correctly? Or something I still misunderstand, please correct me. Thanks, Huan Yang > of the fundamentals of HVO, I kindly suggest a thorough review of the document > in [3]. > > [2] https://lore.kernel.org/lkml/5229b24f-1984-4225-ae03-8b952de56e3b@vivo.com/#t > [3] Documentation/mm/vmemmap_dedup.rst > >> [1] https://lore.kernel.org/linux-mm/20250327092922.536-1-link@vivo.com/T/#m055b34978cf882fd44d2d08d929b50292d8502b4 >> >> Thanks, >> Muchun. >> >
> On Apr 7, 2025, at 09:59, Huan Yang <link@vivo.com> wrote: > > > 在 2025/4/4 18:07, Muchun Song 写道: >> >>> On Apr 4, 2025, at 17:38, Muchun Song <muchun.song@linux.dev> wrote: >>> >>> >>> >>>> On Apr 4, 2025, at 17:01, Christoph Hellwig <hch@lst.de> wrote: >>>> >>>> After the btrfs compressed bio discussion I think the hugetlb changes that >>>> skip the tail pages are fundamentally unsafe in the current kernel. >>>> >>>> That is because the bio_vec representation assumes tail pages do exist, so >>>> as soon as you are doing direct I/O that generates a bvec starting beyond >>>> the present head page things will blow up. Other users of bio_vecs might >>>> do the same, but the way the block bio_vecs are generated are very suspect >>>> to that. So we'll first need to sort that out and a few other things >>>> before we can even think of enabling such a feature. >>>> >>> I would like to express my gratitude to Christoph for including me in the >>> thread. I have carefully read the cover letter in [1], which indicates >>> that an issue has arisen due to the improper use of `vmap_pfn()`. I'm >>> wondering if we could consider using `vmap()` instead. In the HVO scenario, >>> the tail struct pages do **exist**, but they are read-only. I've examined >>> the code of `vmap()`, and it appears that it only reads the struct page. >>> Therefore, it seems feasible for us to use `vmap()` (I am not a expert in >>> udmabuf.). Right? >> I believe my stance is correct. I've also reviewed another thread in [2]. >> Allow me to clarify and correct the viewpoints you presented. You stated: >> " >> So by HVO, it also not backed by pages, only contains folio head, each >> tail pfn's page struct go away. >> " >> This statement is entirely inaccurate. The tail pages do not cease to exist; >> rather, they are read-only. For your specific use-case, please use `vmap()` >> to resolve the issue at hand. If you wish to gain a comprehensive understanding > > I see the document give a simple graph to point: > > +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ > | | | 0 | -------------> | 0 | > | | +-----------+ +-----------+ > | | | 1 | -------------> | 1 | > | | +-----------+ +-----------+ > | | | 2 | ----------------^ ^ ^ ^ ^ ^ > | | +-----------+ | | | | | > | | | 3 | ------------------+ | | | | > | | +-----------+ | | | | > | | | 4 | --------------------+ | | | > | PMD | +-----------+ | | | > | level | | 5 | ----------------------+ | | > | mapping | +-----------+ | | > | | | 6 | ------------------------+ | > | | +-----------+ | > | | | 7 | --------------------------+ > | | +-----------+ > | | > | | > | | > +-----------+ > > If I understand correct, each 2-7 tail's page struct is freed, so if I just need map page 2-7, can we use vmap do > > something correctly? The answer is you can. It is essential to distinguish between virtual address (VA) and physical address (PA). The VAs of tail struct pages aren't freed but remapped to the physical page mapped by the VA of the head struct page (since contents of those tail physical pages are the same). Thus, the freed pages are the physical pages mapped by original tail struct pages, not their virtual addresses. Moreover, while it is possible to read the virtual addresses of these tail struct pages, any write operations are prohibited since it is within the realm of acceptability that the kernel is expected to perform write operations solely on the head struct page of a compound head and conduct read operations only on the tail struct pages. BTW, folio infrastructure is also based on this assumption. Thanks, Muchun. > > Or something I still misunderstand, please correct me. > > Thanks, > > Huan Yang > >> of the fundamentals of HVO, I kindly suggest a thorough review of the document >> in [3]. >> >> [2] https://lore.kernel.org/lkml/5229b24f-1984-4225-ae03-8b952de56e3b@vivo.com/#t >> [3] Documentation/mm/vmemmap_dedup.rst >> >>> [1] https://lore.kernel.org/linux-mm/20250327092922.536-1-link@vivo.com/T/#m055b34978cf882fd44d2d08d929b50292d8502b4 >>> >>> Thanks, >>> Muchun. >>> >>
在 2025/4/7 10:57, Muchun Song 写道: > >> On Apr 7, 2025, at 09:59, Huan Yang <link@vivo.com> wrote: >> >> >> 在 2025/4/4 18:07, Muchun Song 写道: >>>> On Apr 4, 2025, at 17:38, Muchun Song <muchun.song@linux.dev> wrote: >>>> >>>> >>>> >>>>> On Apr 4, 2025, at 17:01, Christoph Hellwig <hch@lst.de> wrote: >>>>> >>>>> After the btrfs compressed bio discussion I think the hugetlb changes that >>>>> skip the tail pages are fundamentally unsafe in the current kernel. >>>>> >>>>> That is because the bio_vec representation assumes tail pages do exist, so >>>>> as soon as you are doing direct I/O that generates a bvec starting beyond >>>>> the present head page things will blow up. Other users of bio_vecs might >>>>> do the same, but the way the block bio_vecs are generated are very suspect >>>>> to that. So we'll first need to sort that out and a few other things >>>>> before we can even think of enabling such a feature. >>>>> >>>> I would like to express my gratitude to Christoph for including me in the >>>> thread. I have carefully read the cover letter in [1], which indicates >>>> that an issue has arisen due to the improper use of `vmap_pfn()`. I'm >>>> wondering if we could consider using `vmap()` instead. In the HVO scenario, >>>> the tail struct pages do **exist**, but they are read-only. I've examined >>>> the code of `vmap()`, and it appears that it only reads the struct page. >>>> Therefore, it seems feasible for us to use `vmap()` (I am not a expert in >>>> udmabuf.). Right? >>> I believe my stance is correct. I've also reviewed another thread in [2]. >>> Allow me to clarify and correct the viewpoints you presented. You stated: >>> " >>> So by HVO, it also not backed by pages, only contains folio head, each >>> tail pfn's page struct go away. >>> " >>> This statement is entirely inaccurate. The tail pages do not cease to exist; >>> rather, they are read-only. For your specific use-case, please use `vmap()` >>> to resolve the issue at hand. If you wish to gain a comprehensive understanding >> I see the document give a simple graph to point: >> >> +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ >> | | | 0 | -------------> | 0 | >> | | +-----------+ +-----------+ >> | | | 1 | -------------> | 1 | >> | | +-----------+ +-----------+ >> | | | 2 | ----------------^ ^ ^ ^ ^ ^ >> | | +-----------+ | | | | | >> | | | 3 | ------------------+ | | | | >> | | +-----------+ | | | | >> | | | 4 | --------------------+ | | | >> | PMD | +-----------+ | | | >> | level | | 5 | ----------------------+ | | >> | mapping | +-----------+ | | >> | | | 6 | ------------------------+ | >> | | +-----------+ | >> | | | 7 | --------------------------+ >> | | +-----------+ >> | | >> | | >> | | >> +-----------+ >> >> If I understand correct, each 2-7 tail's page struct is freed, so if I just need map page 2-7, can we use vmap do >> >> something correctly? > The answer is you can. It is essential to distinguish between virtual Thanks for your reply, but I still can't understand it. For example, I need vmap a hugetlb HVO folio's 2-7 page: struct page **pages = kvmalloc(sizeof(*pages), 6, GFP_KENREL); for (i = 2; i < 8; ++i) pages[i] = folio_page(folio, i); //set 2-7 range page into pages, void *vaddr = vmap(pages, 6, 0, PAGE_KERNEL); For no HVO pages, this can work. If HVO enabled, do "pages[i] = folio_page(folio, i);" just got the head page? and how vmap can correctly map each page? Please correct me. :) Thanks, Huan Yang > address (VA) and physical address (PA). The VAs of tail struct pages > aren't freed but remapped to the physical page mapped by the VA of the > head struct page (since contents of those tail physical pages are the > same). Thus, the freed pages are the physical pages mapped by original > tail struct pages, not their virtual addresses. Moreover, while it > is possible to read the virtual addresses of these tail struct pages, > any write operations are prohibited since it is within the realm of > acceptability that the kernel is expected to perform write operations > solely on the head struct page of a compound head and conduct read > operations only on the tail struct pages. BTW, folio infrastructure > is also based on this assumption. > > Thanks, > Muchun. > >> Or something I still misunderstand, please correct me. >> >> Thanks, >> >> Huan Yang >> >>> of the fundamentals of HVO, I kindly suggest a thorough review of the document >>> in [3]. >>> >>> [2] https://lore.kernel.org/lkml/5229b24f-1984-4225-ae03-8b952de56e3b@vivo.com/#t >>> [3] Documentation/mm/vmemmap_dedup.rst >>> >>>> [1] https://lore.kernel.org/linux-mm/20250327092922.536-1-link@vivo.com/T/#m055b34978cf882fd44d2d08d929b50292d8502b4 >>>> >>>> Thanks, >>>> Muchun. >>>> >
> On Apr 7, 2025, at 11:21, Huan Yang <link@vivo.com> wrote: > > > 在 2025/4/7 10:57, Muchun Song 写道: >> >>> On Apr 7, 2025, at 09:59, Huan Yang <link@vivo.com> wrote: >>> >>> >>> 在 2025/4/4 18:07, Muchun Song 写道: >>>>> On Apr 4, 2025, at 17:38, Muchun Song <muchun.song@linux.dev> wrote: >>>>> >>>>> >>>>> >>>>>> On Apr 4, 2025, at 17:01, Christoph Hellwig <hch@lst.de> wrote: >>>>>> >>>>>> After the btrfs compressed bio discussion I think the hugetlb changes that >>>>>> skip the tail pages are fundamentally unsafe in the current kernel. >>>>>> >>>>>> That is because the bio_vec representation assumes tail pages do exist, so >>>>>> as soon as you are doing direct I/O that generates a bvec starting beyond >>>>>> the present head page things will blow up. Other users of bio_vecs might >>>>>> do the same, but the way the block bio_vecs are generated are very suspect >>>>>> to that. So we'll first need to sort that out and a few other things >>>>>> before we can even think of enabling such a feature. >>>>>> >>>>> I would like to express my gratitude to Christoph for including me in the >>>>> thread. I have carefully read the cover letter in [1], which indicates >>>>> that an issue has arisen due to the improper use of `vmap_pfn()`. I'm >>>>> wondering if we could consider using `vmap()` instead. In the HVO scenario, >>>>> the tail struct pages do **exist**, but they are read-only. I've examined >>>>> the code of `vmap()`, and it appears that it only reads the struct page. >>>>> Therefore, it seems feasible for us to use `vmap()` (I am not a expert in >>>>> udmabuf.). Right? >>>> I believe my stance is correct. I've also reviewed another thread in [2]. >>>> Allow me to clarify and correct the viewpoints you presented. You stated: >>>> " >>>> So by HVO, it also not backed by pages, only contains folio head, each >>>> tail pfn's page struct go away. >>>> " >>>> This statement is entirely inaccurate. The tail pages do not cease to exist; >>>> rather, they are read-only. For your specific use-case, please use `vmap()` >>>> to resolve the issue at hand. If you wish to gain a comprehensive understanding >>> I see the document give a simple graph to point: >>> >>> +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ >>> | | | 0 | -------------> | 0 | >>> | | +-----------+ +-----------+ >>> | | | 1 | -------------> | 1 | >>> | | +-----------+ +-----------+ >>> | | | 2 | ----------------^ ^ ^ ^ ^ ^ >>> | | +-----------+ | | | | | >>> | | | 3 | ------------------+ | | | | >>> | | +-----------+ | | | | >>> | | | 4 | --------------------+ | | | >>> | PMD | +-----------+ | | | >>> | level | | 5 | ----------------------+ | | >>> | mapping | +-----------+ | | >>> | | | 6 | ------------------------+ | >>> | | +-----------+ | >>> | | | 7 | --------------------------+ >>> | | +-----------+ >>> | | >>> | | >>> | | >>> +-----------+ >>> >>> If I understand correct, each 2-7 tail's page struct is freed, so if I just need map page 2-7, can we use vmap do >>> >>> something correctly? >> The answer is you can. It is essential to distinguish between virtual > > Thanks for your reply, but I still can't understand it. For example, I need vmap a hugetlb HVO folio's > > 2-7 page: > > struct page **pages = kvmalloc(sizeof(*pages), 6, GFP_KENREL); > > for (i = 2; i < 8; ++i) > > pages[i] = folio_page(folio, i); //set 2-7 range page into pages, > > void *vaddr = vmap(pages, 6, 0, PAGE_KERNEL); > > For no HVO pages, this can work. If HVO enabled, do "pages[i] = folio_page(folio, i);" just > > got the head page? and how vmap can correctly map each page? Why do you think folio_page(folio, i) (i ≠ 0) returns the head page? Is it speculation or tested? Please base it on the actual situation instead of indulging in wild thoughts. Thanks, Muchun. > > Please correct me. :) > > Thanks, > > Huan Yang > >> address (VA) and physical address (PA). The VAs of tail struct pages >> aren't freed but remapped to the physical page mapped by the VA of the >> head struct page (since contents of those tail physical pages are the >> same). Thus, the freed pages are the physical pages mapped by original >> tail struct pages, not their virtual addresses. Moreover, while it >> is possible to read the virtual addresses of these tail struct pages, >> any write operations are prohibited since it is within the realm of >> acceptability that the kernel is expected to perform write operations >> solely on the head struct page of a compound head and conduct read >> operations only on the tail struct pages. BTW, folio infrastructure >> is also based on this assumption. >> >> Thanks, >> Muchun. >> >>> Or something I still misunderstand, please correct me. >>> >>> Thanks, >>> >>> Huan Yang >>> >>>> of the fundamentals of HVO, I kindly suggest a thorough review of the document >>>> in [3]. >>>> >>>> [2] https://lore.kernel.org/lkml/5229b24f-1984-4225-ae03-8b952de56e3b@vivo.com/#t >>>> [3] Documentation/mm/vmemmap_dedup.rst >>>> >>>>> [1] https://lore.kernel.org/linux-mm/20250327092922.536-1-link@vivo.com/T/#m055b34978cf882fd44d2d08d929b50292d8502b4 >>>>> >>>>> Thanks, >>>>> Muchun.
> On Apr 7, 2025, at 11:37, Muchun Song <muchun.song@linux.dev> wrote: > > > >> On Apr 7, 2025, at 11:21, Huan Yang <link@vivo.com> wrote: >> >> >> 在 2025/4/7 10:57, Muchun Song 写道: >>> >>>> On Apr 7, 2025, at 09:59, Huan Yang <link@vivo.com> wrote: >>>> >>>> >>>> 在 2025/4/4 18:07, Muchun Song 写道: >>>>>> On Apr 4, 2025, at 17:38, Muchun Song <muchun.song@linux.dev> wrote: >>>>>> >>>>>> >>>>>> >>>>>>> On Apr 4, 2025, at 17:01, Christoph Hellwig <hch@lst.de> wrote: >>>>>>> >>>>>>> After the btrfs compressed bio discussion I think the hugetlb changes that >>>>>>> skip the tail pages are fundamentally unsafe in the current kernel. >>>>>>> >>>>>>> That is because the bio_vec representation assumes tail pages do exist, so >>>>>>> as soon as you are doing direct I/O that generates a bvec starting beyond >>>>>>> the present head page things will blow up. Other users of bio_vecs might >>>>>>> do the same, but the way the block bio_vecs are generated are very suspect >>>>>>> to that. So we'll first need to sort that out and a few other things >>>>>>> before we can even think of enabling such a feature. >>>>>>> >>>>>> I would like to express my gratitude to Christoph for including me in the >>>>>> thread. I have carefully read the cover letter in [1], which indicates >>>>>> that an issue has arisen due to the improper use of `vmap_pfn()`. I'm >>>>>> wondering if we could consider using `vmap()` instead. In the HVO scenario, >>>>>> the tail struct pages do **exist**, but they are read-only. I've examined >>>>>> the code of `vmap()`, and it appears that it only reads the struct page. >>>>>> Therefore, it seems feasible for us to use `vmap()` (I am not a expert in >>>>>> udmabuf.). Right? >>>>> I believe my stance is correct. I've also reviewed another thread in [2]. >>>>> Allow me to clarify and correct the viewpoints you presented. You stated: >>>>> " >>>>> So by HVO, it also not backed by pages, only contains folio head, each >>>>> tail pfn's page struct go away. >>>>> " >>>>> This statement is entirely inaccurate. The tail pages do not cease to exist; >>>>> rather, they are read-only. For your specific use-case, please use `vmap()` >>>>> to resolve the issue at hand. If you wish to gain a comprehensive understanding >>>> I see the document give a simple graph to point: >>>> >>>> +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ >>>> | | | 0 | -------------> | 0 | >>>> | | +-----------+ +-----------+ >>>> | | | 1 | -------------> | 1 | >>>> | | +-----------+ +-----------+ >>>> | | | 2 | ----------------^ ^ ^ ^ ^ ^ >>>> | | +-----------+ | | | | | >>>> | | | 3 | ------------------+ | | | | >>>> | | +-----------+ | | | | >>>> | | | 4 | --------------------+ | | | >>>> | PMD | +-----------+ | | | >>>> | level | | 5 | ----------------------+ | | >>>> | mapping | +-----------+ | | >>>> | | | 6 | ------------------------+ | >>>> | | +-----------+ | >>>> | | | 7 | --------------------------+ >>>> | | +-----------+ >>>> | | >>>> | | >>>> | | >>>> +-----------+ >>>> >>>> If I understand correct, each 2-7 tail's page struct is freed, so if I just need map page 2-7, can we use vmap do >>>> >>>> something correctly? >>> The answer is you can. It is essential to distinguish between virtual >> >> Thanks for your reply, but I still can't understand it. For example, I need vmap a hugetlb HVO folio's >> >> 2-7 page: >> >> struct page **pages = kvmalloc(sizeof(*pages), 6, GFP_KENREL); >> >> for (i = 2; i < 8; ++i) >> >> pages[i] = folio_page(folio, i); //set 2-7 range page into pages, >> >> void *vaddr = vmap(pages, 6, 0, PAGE_KERNEL); >> >> For no HVO pages, this can work. If HVO enabled, do "pages[i] = folio_page(folio, i);" just >> >> got the head page? and how vmap can correctly map each page? > > Why do you think folio_page(folio, i) (i ≠ 0) returns the head page? > Is it speculation or tested? Please base it on the actual situation > instead of indulging in wild thoughts. By the way, in case you truly struggle to comprehend the fundamental aspects of HVO, I would like to summarize for you the user-visible behaviors in comparison to the situation where HVO is disabled. HVO Status Tail Page Structures Head Page Structures Enabled Read-Only (RO) Read-Write (RW) Disabled Read-Write (RW) Read-Write (RW) The sole distinction between the two scenarios lies in whether the tail page structures are allowed to be written or not. Please refrain from getting bogged down in the details of the implementation of HVO. Thanks, Muchun. > > Thanks, > Muchun. > >> >> Please correct me. :) >> >> Thanks, >> >> Huan Yang >> >>> address (VA) and physical address (PA). The VAs of tail struct pages >>> aren't freed but remapped to the physical page mapped by the VA of the >>> head struct page (since contents of those tail physical pages are the >>> same). Thus, the freed pages are the physical pages mapped by original >>> tail struct pages, not their virtual addresses. Moreover, while it >>> is possible to read the virtual addresses of these tail struct pages, >>> any write operations are prohibited since it is within the realm of >>> acceptability that the kernel is expected to perform write operations >>> solely on the head struct page of a compound head and conduct read >>> operations only on the tail struct pages. BTW, folio infrastructure >>> is also based on this assumption. >>> >>> Thanks, >>> Muchun. >>> >>>> Or something I still misunderstand, please correct me. >>>> >>>> Thanks, >>>> >>>> Huan Yang >>>> >>>>> of the fundamentals of HVO, I kindly suggest a thorough review of the document >>>>> in [3]. >>>>> >>>>> [2] https://lore.kernel.org/lkml/5229b24f-1984-4225-ae03-8b952de56e3b@vivo.com/#t >>>>> [3] Documentation/mm/vmemmap_dedup.rst >>>>> >>>>>> [1] https://lore.kernel.org/linux-mm/20250327092922.536-1-link@vivo.com/T/#m055b34978cf882fd44d2d08d929b50292d8502b4 >>>>>> >>>>>> Thanks, >>>>>> Muchun.
在 2025/4/7 14:43, Muchun Song 写道: > >> On Apr 7, 2025, at 11:37, Muchun Song <muchun.song@linux.dev> wrote: >> >> >> >>> On Apr 7, 2025, at 11:21, Huan Yang <link@vivo.com> wrote: >>> >>> >>> 在 2025/4/7 10:57, Muchun Song 写道: >>>>> On Apr 7, 2025, at 09:59, Huan Yang <link@vivo.com> wrote: >>>>> >>>>> >>>>> 在 2025/4/4 18:07, Muchun Song 写道: >>>>>>> On Apr 4, 2025, at 17:38, Muchun Song <muchun.song@linux.dev> wrote: >>>>>>> >>>>>>> >>>>>>> >>>>>>>> On Apr 4, 2025, at 17:01, Christoph Hellwig <hch@lst.de> wrote: >>>>>>>> >>>>>>>> After the btrfs compressed bio discussion I think the hugetlb changes that >>>>>>>> skip the tail pages are fundamentally unsafe in the current kernel. >>>>>>>> >>>>>>>> That is because the bio_vec representation assumes tail pages do exist, so >>>>>>>> as soon as you are doing direct I/O that generates a bvec starting beyond >>>>>>>> the present head page things will blow up. Other users of bio_vecs might >>>>>>>> do the same, but the way the block bio_vecs are generated are very suspect >>>>>>>> to that. So we'll first need to sort that out and a few other things >>>>>>>> before we can even think of enabling such a feature. >>>>>>>> >>>>>>> I would like to express my gratitude to Christoph for including me in the >>>>>>> thread. I have carefully read the cover letter in [1], which indicates >>>>>>> that an issue has arisen due to the improper use of `vmap_pfn()`. I'm >>>>>>> wondering if we could consider using `vmap()` instead. In the HVO scenario, >>>>>>> the tail struct pages do **exist**, but they are read-only. I've examined >>>>>>> the code of `vmap()`, and it appears that it only reads the struct page. >>>>>>> Therefore, it seems feasible for us to use `vmap()` (I am not a expert in >>>>>>> udmabuf.). Right? >>>>>> I believe my stance is correct. I've also reviewed another thread in [2]. >>>>>> Allow me to clarify and correct the viewpoints you presented. You stated: >>>>>> " >>>>>> So by HVO, it also not backed by pages, only contains folio head, each >>>>>> tail pfn's page struct go away. >>>>>> " >>>>>> This statement is entirely inaccurate. The tail pages do not cease to exist; >>>>>> rather, they are read-only. For your specific use-case, please use `vmap()` >>>>>> to resolve the issue at hand. If you wish to gain a comprehensive understanding >>>>> I see the document give a simple graph to point: >>>>> >>>>> +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ >>>>> | | | 0 | -------------> | 0 | >>>>> | | +-----------+ +-----------+ >>>>> | | | 1 | -------------> | 1 | >>>>> | | +-----------+ +-----------+ >>>>> | | | 2 | ----------------^ ^ ^ ^ ^ ^ >>>>> | | +-----------+ | | | | | >>>>> | | | 3 | ------------------+ | | | | >>>>> | | +-----------+ | | | | >>>>> | | | 4 | --------------------+ | | | >>>>> | PMD | +-----------+ | | | >>>>> | level | | 5 | ----------------------+ | | >>>>> | mapping | +-----------+ | | >>>>> | | | 6 | ------------------------+ | >>>>> | | +-----------+ | >>>>> | | | 7 | --------------------------+ >>>>> | | +-----------+ >>>>> | | >>>>> | | >>>>> | | >>>>> +-----------+ >>>>> >>>>> If I understand correct, each 2-7 tail's page struct is freed, so if I just need map page 2-7, can we use vmap do >>>>> >>>>> something correctly? >>>> The answer is you can. It is essential to distinguish between virtual >>> Thanks for your reply, but I still can't understand it. For example, I need vmap a hugetlb HVO folio's >>> >>> 2-7 page: >>> >>> struct page **pages = kvmalloc(sizeof(*pages), 6, GFP_KENREL); >>> >>> for (i = 2; i < 8; ++i) >>> >>> pages[i] = folio_page(folio, i); //set 2-7 range page into pages, >>> >>> void *vaddr = vmap(pages, 6, 0, PAGE_KERNEL); >>> >>> For no HVO pages, this can work. If HVO enabled, do "pages[i] = folio_page(folio, i);" just >>> >>> got the head page? and how vmap can correctly map each page? >> Why do you think folio_page(folio, i) (i ≠ 0) returns the head page? >> Is it speculation or tested? Please base it on the actual situation >> instead of indulging in wild thoughts. > By the way, in case you truly struggle to comprehend the fundamental > aspects of HVO, I would like to summarize for you the user-visible > behaviors in comparison to the situation where HVO is disabled. > > HVO Status Tail Page Structures Head Page Structures > Enabled Read-Only (RO) Read-Write (RW) > Disabled Read-Write (RW) Read-Write (RW) > > The sole distinction between the two scenarios lies in whether the > tail page structures are allowed to be written or not. Please refrain > from getting bogged down in the details of the implementation of HVO. Thanks, I do a test, an figure out that I'm totally misunderstand it. Even if HVO enabled, tail page struct freed and point to head, linear mapping still exist, so that any page_to_pfn, page_to_virt(also folio's version), if start from head page can compute each need page like folio_page, can still work: hvo head 0xfffff9de849d0000, pfn=0x127400, wish offset_pfn 0x1275f1, idx 497 is 0xfffff9de849d7c40, pfn=0x1275f1. When vmap, we no need to touch actually page's content, just turn to pfn, so, work well. BTW, even if we need to touch actually input page struct, it point to head page, I guess will effect nothing. If anything still misunderstand, please corrent me. :) Muchun, thank you for your patience, Huan Yang > > Thanks, > Muchun. > >> Thanks, >> Muchun. >> >>> Please correct me. :) >>> >>> Thanks, >>> >>> Huan Yang >>> >>>> address (VA) and physical address (PA). The VAs of tail struct pages >>>> aren't freed but remapped to the physical page mapped by the VA of the >>>> head struct page (since contents of those tail physical pages are the >>>> same). Thus, the freed pages are the physical pages mapped by original >>>> tail struct pages, not their virtual addresses. Moreover, while it >>>> is possible to read the virtual addresses of these tail struct pages, >>>> any write operations are prohibited since it is within the realm of >>>> acceptability that the kernel is expected to perform write operations >>>> solely on the head struct page of a compound head and conduct read >>>> operations only on the tail struct pages. BTW, folio infrastructure >>>> is also based on this assumption. >>>> >>>> Thanks, >>>> Muchun. >>>> >>>>> Or something I still misunderstand, please correct me. >>>>> >>>>> Thanks, >>>>> >>>>> Huan Yang >>>>> >>>>>> of the fundamentals of HVO, I kindly suggest a thorough review of the document >>>>>> in [3]. >>>>>> >>>>>> [2] https://lore.kernel.org/lkml/5229b24f-1984-4225-ae03-8b952de56e3b@vivo.com/#t >>>>>> [3] Documentation/mm/vmemmap_dedup.rst >>>>>> >>>>>>> [1] https://lore.kernel.org/linux-mm/20250327092922.536-1-link@vivo.com/T/#m055b34978cf882fd44d2d08d929b50292d8502b4 >>>>>>> >>>>>>> Thanks, >>>>>>> Muchun. > >
> On Apr 7, 2025, at 15:09, Huan Yang <link@vivo.com> wrote: > > > 在 2025/4/7 14:43, Muchun Song 写道: >> >>> On Apr 7, 2025, at 11:37, Muchun Song <muchun.song@linux.dev> wrote: >>> >>> >>> >>>> On Apr 7, 2025, at 11:21, Huan Yang <link@vivo.com> wrote: >>>> >>>> >>>> 在 2025/4/7 10:57, Muchun Song 写道: >>>>>> On Apr 7, 2025, at 09:59, Huan Yang <link@vivo.com> wrote: >>>>>> >>>>>> >>>>>> 在 2025/4/4 18:07, Muchun Song 写道: >>>>>>>> On Apr 4, 2025, at 17:38, Muchun Song <muchun.song@linux.dev> wrote: >>>>>>>> >>>>>>>> >>>>>>>> >>>>>>>>> On Apr 4, 2025, at 17:01, Christoph Hellwig <hch@lst.de> wrote: >>>>>>>>> >>>>>>>>> After the btrfs compressed bio discussion I think the hugetlb changes that >>>>>>>>> skip the tail pages are fundamentally unsafe in the current kernel. >>>>>>>>> >>>>>>>>> That is because the bio_vec representation assumes tail pages do exist, so >>>>>>>>> as soon as you are doing direct I/O that generates a bvec starting beyond >>>>>>>>> the present head page things will blow up. Other users of bio_vecs might >>>>>>>>> do the same, but the way the block bio_vecs are generated are very suspect >>>>>>>>> to that. So we'll first need to sort that out and a few other things >>>>>>>>> before we can even think of enabling such a feature. >>>>>>>>> >>>>>>>> I would like to express my gratitude to Christoph for including me in the >>>>>>>> thread. I have carefully read the cover letter in [1], which indicates >>>>>>>> that an issue has arisen due to the improper use of `vmap_pfn()`. I'm >>>>>>>> wondering if we could consider using `vmap()` instead. In the HVO scenario, >>>>>>>> the tail struct pages do **exist**, but they are read-only. I've examined >>>>>>>> the code of `vmap()`, and it appears that it only reads the struct page. >>>>>>>> Therefore, it seems feasible for us to use `vmap()` (I am not a expert in >>>>>>>> udmabuf.). Right? >>>>>>> I believe my stance is correct. I've also reviewed another thread in [2]. >>>>>>> Allow me to clarify and correct the viewpoints you presented. You stated: >>>>>>> " >>>>>>> So by HVO, it also not backed by pages, only contains folio head, each >>>>>>> tail pfn's page struct go away. >>>>>>> " >>>>>>> This statement is entirely inaccurate. The tail pages do not cease to exist; >>>>>>> rather, they are read-only. For your specific use-case, please use `vmap()` >>>>>>> to resolve the issue at hand. If you wish to gain a comprehensive understanding >>>>>> I see the document give a simple graph to point: >>>>>> >>>>>> +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ >>>>>> | | | 0 | -------------> | 0 | >>>>>> | | +-----------+ +-----------+ >>>>>> | | | 1 | -------------> | 1 | >>>>>> | | +-----------+ +-----------+ >>>>>> | | | 2 | ----------------^ ^ ^ ^ ^ ^ >>>>>> | | +-----------+ | | | | | >>>>>> | | | 3 | ------------------+ | | | | >>>>>> | | +-----------+ | | | | >>>>>> | | | 4 | --------------------+ | | | >>>>>> | PMD | +-----------+ | | | >>>>>> | level | | 5 | ----------------------+ | | >>>>>> | mapping | +-----------+ | | >>>>>> | | | 6 | ------------------------+ | >>>>>> | | +-----------+ | >>>>>> | | | 7 | --------------------------+ >>>>>> | | +-----------+ >>>>>> | | >>>>>> | | >>>>>> | | >>>>>> +-----------+ >>>>>> >>>>>> If I understand correct, each 2-7 tail's page struct is freed, so if I just need map page 2-7, can we use vmap do >>>>>> >>>>>> something correctly? >>>>> The answer is you can. It is essential to distinguish between virtual >>>> Thanks for your reply, but I still can't understand it. For example, I need vmap a hugetlb HVO folio's >>>> >>>> 2-7 page: >>>> >>>> struct page **pages = kvmalloc(sizeof(*pages), 6, GFP_KENREL); >>>> >>>> for (i = 2; i < 8; ++i) >>>> >>>> pages[i] = folio_page(folio, i); //set 2-7 range page into pages, >>>> >>>> void *vaddr = vmap(pages, 6, 0, PAGE_KERNEL); >>>> >>>> For no HVO pages, this can work. If HVO enabled, do "pages[i] = folio_page(folio, i);" just >>>> >>>> got the head page? and how vmap can correctly map each page? >>> Why do you think folio_page(folio, i) (i ≠ 0) returns the head page? >>> Is it speculation or tested? Please base it on the actual situation >>> instead of indulging in wild thoughts. >> By the way, in case you truly struggle to comprehend the fundamental >> aspects of HVO, I would like to summarize for you the user-visible >> behaviors in comparison to the situation where HVO is disabled. >> >> HVO Status Tail Page Structures Head Page Structures >> Enabled Read-Only (RO) Read-Write (RW) >> Disabled Read-Write (RW) Read-Write (RW) >> >> The sole distinction between the two scenarios lies in whether the >> tail page structures are allowed to be written or not. Please refrain >> from getting bogged down in the details of the implementation of HVO. > > Thanks, I do a test, an figure out that I'm totally misunderstand it. > > Even if HVO enabled, tail page struct freed and point to head, linear mapping still exist, so that any page_to_pfn, > > page_to_virt(also folio's version), if start from head page can compute each need page like folio_page, can still work: > > hvo head 0xfffff9de849d0000, pfn=0x127400, wish offset_pfn 0x1275f1, idx 497 is 0xfffff9de849d7c40, pfn=0x1275f1. > > When vmap, we no need to touch actually page's content, just turn to pfn, so, work well. You are able to read those tail page structures. The reason why vmap can function is not that it doesn't read those page structures. What I mean is that vmap will still work even if it does read the page structures, because those tail page structures do indeed exist. > > BTW, even if we need to touch actually input page struct, it point to head page, I guess will effect nothing. Allow me to clarify this for you to ensure that we have a shared understanding. Those tail page structures (virtual addresses in the vmemmap area) are mapped to the same page frame (physical page) to which the head page structures (virtual addresses in the vmemmap area) are mapped. It is analogous to the shared-mapping mechanism in the user space. > > If anything still misunderstand, please corrent me. :) > > Muchun, thank you for your patience, > > Huan Yang > >> >> Thanks, >> Muchun. >> >>> Thanks, >>> Muchun. >>> >>>> Please correct me. :) >>>> >>>> Thanks, >>>> >>>> Huan Yang >>>> >>>>> address (VA) and physical address (PA). The VAs of tail struct pages >>>>> aren't freed but remapped to the physical page mapped by the VA of the >>>>> head struct page (since contents of those tail physical pages are the >>>>> same). Thus, the freed pages are the physical pages mapped by original >>>>> tail struct pages, not their virtual addresses. Moreover, while it >>>>> is possible to read the virtual addresses of these tail struct pages, >>>>> any write operations are prohibited since it is within the realm of >>>>> acceptability that the kernel is expected to perform write operations >>>>> solely on the head struct page of a compound head and conduct read >>>>> operations only on the tail struct pages. BTW, folio infrastructure >>>>> is also based on this assumption. >>>>> >>>>> Thanks, >>>>> Muchun. >>>>> >>>>>> Or something I still misunderstand, please correct me. >>>>>> >>>>>> Thanks, >>>>>> >>>>>> Huan Yang >>>>>> >>>>>>> of the fundamentals of HVO, I kindly suggest a thorough review of the document >>>>>>> in [3]. >>>>>>> >>>>>>> [2] https://lore.kernel.org/lkml/5229b24f-1984-4225-ae03-8b952de56e3b@vivo.com/#t >>>>>>> [3] Documentation/mm/vmemmap_dedup.rst >>>>>>> >>>>>>>> [1] https://lore.kernel.org/linux-mm/20250327092922.536-1-link@vivo.com/T/#m055b34978cf882fd44d2d08d929b50292d8502b4 >>>>>>>> >>>>>>>> Thanks, >>>>>>>> Muchun.
在 2025/4/7 15:22, Muchun Song 写道: > >> On Apr 7, 2025, at 15:09, Huan Yang <link@vivo.com> wrote: >> >> >> 在 2025/4/7 14:43, Muchun Song 写道: >>>> On Apr 7, 2025, at 11:37, Muchun Song <muchun.song@linux.dev> wrote: >>>> >>>> >>>> >>>>> On Apr 7, 2025, at 11:21, Huan Yang <link@vivo.com> wrote: >>>>> >>>>> >>>>> 在 2025/4/7 10:57, Muchun Song 写道: >>>>>>> On Apr 7, 2025, at 09:59, Huan Yang <link@vivo.com> wrote: >>>>>>> >>>>>>> >>>>>>> 在 2025/4/4 18:07, Muchun Song 写道: >>>>>>>>> On Apr 4, 2025, at 17:38, Muchun Song <muchun.song@linux.dev> wrote: >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>>> On Apr 4, 2025, at 17:01, Christoph Hellwig <hch@lst.de> wrote: >>>>>>>>>> >>>>>>>>>> After the btrfs compressed bio discussion I think the hugetlb changes that >>>>>>>>>> skip the tail pages are fundamentally unsafe in the current kernel. >>>>>>>>>> >>>>>>>>>> That is because the bio_vec representation assumes tail pages do exist, so >>>>>>>>>> as soon as you are doing direct I/O that generates a bvec starting beyond >>>>>>>>>> the present head page things will blow up. Other users of bio_vecs might >>>>>>>>>> do the same, but the way the block bio_vecs are generated are very suspect >>>>>>>>>> to that. So we'll first need to sort that out and a few other things >>>>>>>>>> before we can even think of enabling such a feature. >>>>>>>>>> >>>>>>>>> I would like to express my gratitude to Christoph for including me in the >>>>>>>>> thread. I have carefully read the cover letter in [1], which indicates >>>>>>>>> that an issue has arisen due to the improper use of `vmap_pfn()`. I'm >>>>>>>>> wondering if we could consider using `vmap()` instead. In the HVO scenario, >>>>>>>>> the tail struct pages do **exist**, but they are read-only. I've examined >>>>>>>>> the code of `vmap()`, and it appears that it only reads the struct page. >>>>>>>>> Therefore, it seems feasible for us to use `vmap()` (I am not a expert in >>>>>>>>> udmabuf.). Right? >>>>>>>> I believe my stance is correct. I've also reviewed another thread in [2]. >>>>>>>> Allow me to clarify and correct the viewpoints you presented. You stated: >>>>>>>> " >>>>>>>> So by HVO, it also not backed by pages, only contains folio head, each >>>>>>>> tail pfn's page struct go away. >>>>>>>> " >>>>>>>> This statement is entirely inaccurate. The tail pages do not cease to exist; >>>>>>>> rather, they are read-only. For your specific use-case, please use `vmap()` >>>>>>>> to resolve the issue at hand. If you wish to gain a comprehensive understanding >>>>>>> I see the document give a simple graph to point: >>>>>>> >>>>>>> +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ >>>>>>> | | | 0 | -------------> | 0 | >>>>>>> | | +-----------+ +-----------+ >>>>>>> | | | 1 | -------------> | 1 | >>>>>>> | | +-----------+ +-----------+ >>>>>>> | | | 2 | ----------------^ ^ ^ ^ ^ ^ >>>>>>> | | +-----------+ | | | | | >>>>>>> | | | 3 | ------------------+ | | | | >>>>>>> | | +-----------+ | | | | >>>>>>> | | | 4 | --------------------+ | | | >>>>>>> | PMD | +-----------+ | | | >>>>>>> | level | | 5 | ----------------------+ | | >>>>>>> | mapping | +-----------+ | | >>>>>>> | | | 6 | ------------------------+ | >>>>>>> | | +-----------+ | >>>>>>> | | | 7 | --------------------------+ >>>>>>> | | +-----------+ >>>>>>> | | >>>>>>> | | >>>>>>> | | >>>>>>> +-----------+ >>>>>>> >>>>>>> If I understand correct, each 2-7 tail's page struct is freed, so if I just need map page 2-7, can we use vmap do >>>>>>> >>>>>>> something correctly? >>>>>> The answer is you can. It is essential to distinguish between virtual >>>>> Thanks for your reply, but I still can't understand it. For example, I need vmap a hugetlb HVO folio's >>>>> >>>>> 2-7 page: >>>>> >>>>> struct page **pages = kvmalloc(sizeof(*pages), 6, GFP_KENREL); >>>>> >>>>> for (i = 2; i < 8; ++i) >>>>> >>>>> pages[i] = folio_page(folio, i); //set 2-7 range page into pages, >>>>> >>>>> void *vaddr = vmap(pages, 6, 0, PAGE_KERNEL); >>>>> >>>>> For no HVO pages, this can work. If HVO enabled, do "pages[i] = folio_page(folio, i);" just >>>>> >>>>> got the head page? and how vmap can correctly map each page? >>>> Why do you think folio_page(folio, i) (i ≠ 0) returns the head page? >>>> Is it speculation or tested? Please base it on the actual situation >>>> instead of indulging in wild thoughts. >>> By the way, in case you truly struggle to comprehend the fundamental >>> aspects of HVO, I would like to summarize for you the user-visible >>> behaviors in comparison to the situation where HVO is disabled. >>> >>> HVO Status Tail Page Structures Head Page Structures >>> Enabled Read-Only (RO) Read-Write (RW) >>> Disabled Read-Write (RW) Read-Write (RW) >>> >>> The sole distinction between the two scenarios lies in whether the >>> tail page structures are allowed to be written or not. Please refrain >>> from getting bogged down in the details of the implementation of HVO. >> Thanks, I do a test, an figure out that I'm totally misunderstand it. >> >> Even if HVO enabled, tail page struct freed and point to head, linear mapping still exist, so that any page_to_pfn, >> >> page_to_virt(also folio's version), if start from head page can compute each need page like folio_page, can still work: >> >> hvo head 0xfffff9de849d0000, pfn=0x127400, wish offset_pfn 0x1275f1, idx 497 is 0xfffff9de849d7c40, pfn=0x1275f1. >> >> When vmap, we no need to touch actually page's content, just turn to pfn, so, work well. > You are able to read those tail page structures. The reason why vmap can > function is not that it doesn't read those page structures. What I mean > is that vmap will still work even if it does read the page structures, > because those tail page structures do indeed exist. > >> BTW, even if we need to touch actually input page struct, it point to head page, I guess will effect nothing. > Allow me to clarify this for you to ensure that we have a shared understanding. > Those tail page structures (virtual addresses in the vmemmap area) are mapped > to the same page frame (physical page) to which the head page structures (virtual > addresses in the vmemmap area) are mapped. It is analogous to the shared-mapping > mechanism in the user space. Thank you for your answer. I may understand it. HVO do not release vmemmap page struct pointer array, just change it's va point to head page's.(vmemmap_remap_pte) So: 1. any deal of page struct pointer still work, can get right pfn or something. 2. Any read of this va still work, we can get correct folio info, but can't change it.(PAGE_KERNEL_RO) What I misunderstand ahead is vmemmap's page struct pointer also freed, what a fool. :( Thanks, Huan Yang > >> If anything still misunderstand, please corrent me. :) >> >> Muchun, thank you for your patience, >> >> Huan Yang >> >>> Thanks, >>> Muchun. >>> >>>> Thanks, >>>> Muchun. >>>> >>>>> Please correct me. :) >>>>> >>>>> Thanks, >>>>> >>>>> Huan Yang >>>>> >>>>>> address (VA) and physical address (PA). The VAs of tail struct pages >>>>>> aren't freed but remapped to the physical page mapped by the VA of the >>>>>> head struct page (since contents of those tail physical pages are the >>>>>> same). Thus, the freed pages are the physical pages mapped by original >>>>>> tail struct pages, not their virtual addresses. Moreover, while it >>>>>> is possible to read the virtual addresses of these tail struct pages, >>>>>> any write operations are prohibited since it is within the realm of >>>>>> acceptability that the kernel is expected to perform write operations >>>>>> solely on the head struct page of a compound head and conduct read >>>>>> operations only on the tail struct pages. BTW, folio infrastructure >>>>>> is also based on this assumption. >>>>>> >>>>>> Thanks, >>>>>> Muchun. >>>>>> >>>>>>> Or something I still misunderstand, please correct me. >>>>>>> >>>>>>> Thanks, >>>>>>> >>>>>>> Huan Yang >>>>>>> >>>>>>>> of the fundamentals of HVO, I kindly suggest a thorough review of the document >>>>>>>> in [3]. >>>>>>>> >>>>>>>> [2] https://lore.kernel.org/lkml/5229b24f-1984-4225-ae03-8b952de56e3b@vivo.com/#t >>>>>>>> [3] Documentation/mm/vmemmap_dedup.rst >>>>>>>> >>>>>>>>> [1] https://lore.kernel.org/linux-mm/20250327092922.536-1-link@vivo.com/T/#m055b34978cf882fd44d2d08d929b50292d8502b4 >>>>>>>>> >>>>>>>>> Thanks, >>>>>>>>> Muchun. > >
On Mon, Apr 07, 2025 at 02:43:20PM +0800, Muchun Song wrote: > By the way, in case you truly struggle to comprehend the fundamental > aspects of HVO, I would like to summarize for you the user-visible > behaviors in comparison to the situation where HVO is disabled. > > HVO Status Tail Page Structures Head Page Structures > Enabled Read-Only (RO) Read-Write (RW) > Disabled Read-Write (RW) Read-Write (RW) > > The sole distinction between the two scenarios lies in whether the > tail page structures are allowed to be written or not. Please refrain > from getting bogged down in the details of the implementation of HVO. This feels extremely fragile to me. I doubt many people know what operations needs read vs write access to tail pages. Or for higher level operations if needs access to tail pages at all.
> On Apr 7, 2025, at 16:59, Christoph Hellwig <hch@lst.de> wrote: > > On Mon, Apr 07, 2025 at 02:43:20PM +0800, Muchun Song wrote: >> By the way, in case you truly struggle to comprehend the fundamental >> aspects of HVO, I would like to summarize for you the user-visible >> behaviors in comparison to the situation where HVO is disabled. >> >> HVO Status Tail Page Structures Head Page Structures >> Enabled Read-Only (RO) Read-Write (RW) >> Disabled Read-Write (RW) Read-Write (RW) >> >> The sole distinction between the two scenarios lies in whether the >> tail page structures are allowed to be written or not. Please refrain >> from getting bogged down in the details of the implementation of HVO. > > This feels extremely fragile to me. I doubt many people know what > operations needs read vs write access to tail pages. Or for higher > level operations if needs access to tail pages at all. > A compound page should modify its head page structure (e.g., refcount), which is why `compound_head()` is widely used. Modifying its tail page structures is incorrect. Users needn't worry about whether to modify tail page structures. They should use `compound_head(tail)` to get the head page structure and update it. All users must follow this rule (I think folio-infrastructure also requires this). If a user tries to write to a HugeTLB page's tail page, an exception will be raised as these tail pages are read-only mapped to catch invalid operations.