| Message ID | 20201113105952.11638-1-songmuchun@bytedance.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | Free some vmemmap pages of hugetlb page | expand |
> -----Original Message----- > From: owner-linux-mm@kvack.org [mailto:owner-linux-mm@kvack.org] On > Behalf Of Muchun Song > Sent: Saturday, November 14, 2020 12:00 AM > To: corbet@lwn.net; mike.kravetz@oracle.com; tglx@linutronix.de; > mingo@redhat.com; bp@alien8.de; x86@kernel.org; hpa@zytor.com; > dave.hansen@linux.intel.com; luto@kernel.org; peterz@infradead.org; > viro@zeniv.linux.org.uk; akpm@linux-foundation.org; paulmck@kernel.org; > mchehab+huawei@kernel.org; pawan.kumar.gupta@linux.intel.com; > rdunlap@infradead.org; oneukum@suse.com; anshuman.khandual@arm.com; > jroedel@suse.de; almasrymina@google.com; rientjes@google.com; > willy@infradead.org; osalvador@suse.de; mhocko@suse.com > Cc: duanxiongchun@bytedance.com; linux-doc@vger.kernel.org; > linux-kernel@vger.kernel.org; linux-mm@kvack.org; > linux-fsdevel@vger.kernel.org; Muchun Song <songmuchun@bytedance.com> > Subject: [PATCH v4 00/21] Free some vmemmap pages of hugetlb page > > Hi all, > > This patch series will free some vmemmap pages(struct page structures) > associated with each hugetlbpage when preallocated to save memory. > > Nowadays we track the status of physical page frames using struct page > structures arranged in one or more arrays. And here exists one-to-one > mapping between the physical page frame and the corresponding struct page > structure. > > The HugeTLB support is built on top of multiple page size support that > is provided by most modern architectures. For example, x86 CPUs normally > support 4K and 2M (1G if architecturally supported) page sizes. Every > HugeTLB has more than one struct page structure. The 2M HugeTLB has 512 > struct page structure and 1G HugeTLB has 4096 struct page structures. But > in the core of HugeTLB only uses the first 4 (Use of first 4 struct page > structures comes from HUGETLB_CGROUP_MIN_ORDER.) struct page > structures to > store metadata associated with each HugeTLB. The rest of the struct page > structures are usually read the compound_head field which are all the same > value. If we can free some struct page memory to buddy system so that we > can save a lot of memory. > > When the system boot up, every 2M HugeTLB has 512 struct page structures > which size is 8 pages(sizeof(struct page) * 512 / PAGE_SIZE). > > hugetlbpage struct pages(8 pages) page > frame(8 pages) > +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ > | | | 0 | -------------> | 0 > | > | | | 1 | -------------> | 1 > | > | | | 2 | -------------> | 2 > | > | | | 3 | -------------> | 3 > | > | | | 4 | -------------> | 4 > | > | 2M | | 5 | -------------> | > 5 | > | | | 6 | -------------> | 6 > | > | | | 7 | -------------> | 7 > | > | | +-----------+ > +-----------+ > | | > | | > +-----------+ > > > When a hugetlbpage is preallocated, we can change the mapping from above > to > bellow. > > hugetlbpage struct pages(8 pages) page > frame(8 pages) > +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ > | | | 0 | -------------> | 0 > | > | | | 1 | -------------> | 1 > | > | | | 2 | -------------> > +-----------+ > | | | 3 | -----------------^ ^ ^ ^ > ^ > | | | 4 | -------------------+ | | > | > | 2M | | 5 | ---------------------+ | > | > | | | 6 | -----------------------+ | > | | | 7 | -------------------------+ > | | +-----------+ > | | > | | > +-----------+ > > For tail pages, the value of compound_head is the same. So we can reuse > first page of tail page structs. We map the virtual addresses of the > remaining 6 pages of tail page structs to the first tail page struct, > and then free these 6 pages. Therefore, we need to reserve at least 2 > pages as vmemmap areas. > > When a hugetlbpage is freed to the buddy system, we should allocate six > pages for vmemmap pages and restore the previous mapping relationship. > > If we uses the 1G hugetlbpage, we can save 4088 pages(There are 4096 pages > for > struct page structures, we reserve 2 pages for vmemmap and 8 pages for page > tables. So we can save 4088 pages). This is a very substantial gain. On our > server, run some SPDK/QEMU applications which will use 1024GB hugetlbpage. > With this feature enabled, we can save ~16GB(1G hugepage)/~11GB(2MB > hugepage) Hi Muchun, Do we really save 11GB for 2MB hugepage? How much do we save if we only get one 2MB hugetlb from one 128MB mem_section? It seems we need to get at least one page for the PTEs since we are splitting PMD of vmemmap into PTE? > memory. > > Because there are vmemmap page tables reconstruction on the > freeing/allocating > path, it increases some overhead. Here are some overhead analysis. > > 1) Allocating 10240 2MB hugetlb pages. > > a) With this patch series applied: > # time echo 10240 > /proc/sys/vm/nr_hugepages > > real 0m0.166s > user 0m0.000s > sys 0m0.166s > > # bpftrace -e 'kprobe:alloc_fresh_huge_page { @start[tid] = nsecs; } > kretprobe:alloc_fresh_huge_page /@start[tid]/ { @latency = hist(nsecs - > @start[tid]); delete(@start[tid]); }' > Attaching 2 probes... > > @latency: > [8K, 16K) 8360 > |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ > @@@@@@@@@@@@@@@@| > [16K, 32K) 1868 |@@@@@@@@@@@ > | > [32K, 64K) 10 | > | > [64K, 128K) 2 | > | > > b) Without this patch series: > # time echo 10240 > /proc/sys/vm/nr_hugepages > > real 0m0.066s > user 0m0.000s > sys 0m0.066s > > # bpftrace -e 'kprobe:alloc_fresh_huge_page { @start[tid] = nsecs; } > kretprobe:alloc_fresh_huge_page /@start[tid]/ { @latency = hist(nsecs - > @start[tid]); delete(@start[tid]); }' > Attaching 2 probes... > > @latency: > [4K, 8K) 10176 > |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ > @@@@@@@@@@@@@@@@| > [8K, 16K) 62 | > | > [16K, 32K) 2 | > | > > Summarize: this feature is about ~2x slower than before. > > 2) Freeing 10240 @MB hugetlb pages. > > a) With this patch series applied: > # time echo 0 > /proc/sys/vm/nr_hugepages > > real 0m0.004s > user 0m0.000s > sys 0m0.002s > Something is wrong here, it is faster than the case without this patchset: 0.004s vs. 0m0.077s > # bpftrace -e 'kprobe:__free_hugepage { @start[tid] = nsecs; } > kretprobe:__free_hugepage /@start[tid]/ { @latency = hist(nsecs - @start[tid]); > delete(@start[tid]); }' > Attaching 2 probes... > > @latency: > [16K, 32K) 10240 > |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ > @@@@@@@@@@@@@@@@| > > b) Without this patch series: > # time echo 0 > /proc/sys/vm/nr_hugepages > > real 0m0.077s > user 0m0.001s > sys 0m0.075s > > # bpftrace -e 'kprobe:__free_hugepage { @start[tid] = nsecs; } > kretprobe:__free_hugepage /@start[tid]/ { @latency = hist(nsecs - @start[tid]); > delete(@start[tid]); }' > Attaching 2 probes... > > @latency: > [4K, 8K) 9950 > |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ > @@@@@@@@@@@@@@@@| > [8K, 16K) 287 |@ > | > [16K, 32K) 3 | > | > > Summarize: The overhead of __free_hugepage is about ~2-4x slower than > before. > But according to the allocation test above, I think that here is > also ~2x slower than before. > > But why the 'real' time of patched is smaller than before? > Because > In this patch series, the freeing hugetlb is asynchronous(through > kwoker). > > Although the overhead has increased, the overhead is not significant. Like MIke > said, "However, remember that the majority of use cases create hugetlb pages > at > or shortly after boot time and add them to the pool. So, additional overhead is > at pool creation time. There is no change to 'normal run time' operations of > getting a page from or returning a page to the pool (think page fault/unmap)". > It seems it is true. At runtime, people normally don't change hugetlb. > changelog in v4: > 1. Move all the vmemmap functions to hugetlb_vmemmap.c. > 2. Make the CONFIG_HUGETLB_PAGE_FREE_VMEMMAP default to y, if we > want to > disable this feature, we should disable it by a boot/kernel command line. > 3. Remove vmemmap_pgtable_{init, deposit, withdraw}() helper functions. > 4. Initialize page table lock for vmemmap through core_initcall mechanism. > > Thanks for Mike and Oscar's suggestions. > > changelog in v3: > 1. Rename some helps function name. Thanks Mike. > 2. Rework some code. Thanks Mike and Oscar. > 3. Remap the tail vmemmap page with PAGE_KERNEL_RO instead of > PAGE_KERNEL. Thanks Matthew. > 4. Add some overhead analysis in the cover letter. > 5. Use vmemap pmd table lock instead of a hugetlb specific global lock. > > changelog in v2: > 1. Fix do not call dissolve_compound_page in alloc_huge_page_vmemmap(). > 2. Fix some typo and code style problems. > 3. Remove unused handle_vmemmap_fault(). > 4. Merge some commits to one commit suggested by Mike. > > Muchun Song (21): > mm/memory_hotplug: Move bootmem info registration API to > bootmem_info.c > mm/memory_hotplug: Move {get,put}_page_bootmem() to bootmem_info.c > mm/hugetlb: Introduce a new config HUGETLB_PAGE_FREE_VMEMMAP > mm/hugetlb: Introduce nr_free_vmemmap_pages in the struct hstate > mm/hugetlb: Introduce pgtable allocation/freeing helpers > mm/bootmem_info: Introduce {free,prepare}_vmemmap_page() > mm/bootmem_info: Combine bootmem info and type into page->freelist > mm/hugetlb: Initialize page table lock for vmemmap > mm/hugetlb: Free the vmemmap pages associated with each hugetlb page > mm/hugetlb: Defer freeing of hugetlb pages > mm/hugetlb: Allocate the vmemmap pages associated with each hugetlb > page > mm/hugetlb: Introduce remap_huge_page_pmd_vmemmap helper > mm/hugetlb: Use PG_slab to indicate split pmd > mm/hugetlb: Support freeing vmemmap pages of gigantic page > mm/hugetlb: Set the PageHWPoison to the raw error page > mm/hugetlb: Flush work when dissolving hugetlb page > mm/hugetlb: Add a kernel parameter hugetlb_free_vmemmap > mm/hugetlb: Merge pte to huge pmd only for gigantic page > mm/hugetlb: Gather discrete indexes of tail page > mm/hugetlb: Add BUILD_BUG_ON to catch invalid usage of tail struct > page > mm/hugetlb: Disable freeing vmemmap if struct page size is not power > of two > > Documentation/admin-guide/kernel-parameters.txt | 9 + > Documentation/admin-guide/mm/hugetlbpage.rst | 3 + > arch/x86/include/asm/hugetlb.h | 17 + > arch/x86/include/asm/pgtable_64_types.h | 8 + > arch/x86/mm/init_64.c | 7 +- > fs/Kconfig | 14 + > include/linux/bootmem_info.h | 78 +++ > include/linux/hugetlb.h | 19 + > include/linux/hugetlb_cgroup.h | 15 +- > include/linux/memory_hotplug.h | 27 - > mm/Makefile | 2 + > mm/bootmem_info.c | 124 ++++ > mm/hugetlb.c | 163 +++++- > mm/hugetlb_vmemmap.c | 732 > ++++++++++++++++++++++++ > mm/hugetlb_vmemmap.h | 104 ++++ > mm/memory_hotplug.c | 116 ---- > mm/sparse.c | 5 +- > 17 files changed, 1263 insertions(+), 180 deletions(-) > create mode 100644 include/linux/bootmem_info.h > create mode 100644 mm/bootmem_info.c > create mode 100644 mm/hugetlb_vmemmap.c > create mode 100644 mm/hugetlb_vmemmap.h > Thanks Barry
On Tue, Nov 17, 2020 at 6:16 PM Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com> wrote: > > > > > -----Original Message----- > > From: owner-linux-mm@kvack.org [mailto:owner-linux-mm@kvack.org] On > > Behalf Of Muchun Song > > Sent: Saturday, November 14, 2020 12:00 AM > > To: corbet@lwn.net; mike.kravetz@oracle.com; tglx@linutronix.de; > > mingo@redhat.com; bp@alien8.de; x86@kernel.org; hpa@zytor.com; > > dave.hansen@linux.intel.com; luto@kernel.org; peterz@infradead.org; > > viro@zeniv.linux.org.uk; akpm@linux-foundation.org; paulmck@kernel.org; > > mchehab+huawei@kernel.org; pawan.kumar.gupta@linux.intel.com; > > rdunlap@infradead.org; oneukum@suse.com; anshuman.khandual@arm.com; > > jroedel@suse.de; almasrymina@google.com; rientjes@google.com; > > willy@infradead.org; osalvador@suse.de; mhocko@suse.com > > Cc: duanxiongchun@bytedance.com; linux-doc@vger.kernel.org; > > linux-kernel@vger.kernel.org; linux-mm@kvack.org; > > linux-fsdevel@vger.kernel.org; Muchun Song <songmuchun@bytedance.com> > > Subject: [PATCH v4 00/21] Free some vmemmap pages of hugetlb page > > > > Hi all, > > > > This patch series will free some vmemmap pages(struct page structures) > > associated with each hugetlbpage when preallocated to save memory. > > > > Nowadays we track the status of physical page frames using struct page > > structures arranged in one or more arrays. And here exists one-to-one > > mapping between the physical page frame and the corresponding struct page > > structure. > > > > The HugeTLB support is built on top of multiple page size support that > > is provided by most modern architectures. For example, x86 CPUs normally > > support 4K and 2M (1G if architecturally supported) page sizes. Every > > HugeTLB has more than one struct page structure. The 2M HugeTLB has 512 > > struct page structure and 1G HugeTLB has 4096 struct page structures. But > > in the core of HugeTLB only uses the first 4 (Use of first 4 struct page > > structures comes from HUGETLB_CGROUP_MIN_ORDER.) struct page > > structures to > > store metadata associated with each HugeTLB. The rest of the struct page > > structures are usually read the compound_head field which are all the same > > value. If we can free some struct page memory to buddy system so that we > > can save a lot of memory. > > > > When the system boot up, every 2M HugeTLB has 512 struct page structures > > which size is 8 pages(sizeof(struct page) * 512 / PAGE_SIZE). > > > > hugetlbpage struct pages(8 pages) page > > frame(8 pages) > > +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ > > | | | 0 | -------------> | 0 > > | > > | | | 1 | -------------> | 1 > > | > > | | | 2 | -------------> | 2 > > | > > | | | 3 | -------------> | 3 > > | > > | | | 4 | -------------> | 4 > > | > > | 2M | | 5 | -------------> | > > 5 | > > | | | 6 | -------------> | 6 > > | > > | | | 7 | -------------> | 7 > > | > > | | +-----------+ > > +-----------+ > > | | > > | | > > +-----------+ > > > > > > When a hugetlbpage is preallocated, we can change the mapping from above > > to > > bellow. > > > > hugetlbpage struct pages(8 pages) page > > frame(8 pages) > > +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ > > | | | 0 | -------------> | 0 > > | > > | | | 1 | -------------> | 1 > > | > > | | | 2 | -------------> > > +-----------+ > > | | | 3 | -----------------^ ^ ^ ^ > > ^ > > | | | 4 | -------------------+ | | > > | > > | 2M | | 5 | ---------------------+ | > > | > > | | | 6 | -----------------------+ | > > | | | 7 | -------------------------+ > > | | +-----------+ > > | | > > | | > > +-----------+ > > > > For tail pages, the value of compound_head is the same. So we can reuse > > first page of tail page structs. We map the virtual addresses of the > > remaining 6 pages of tail page structs to the first tail page struct, > > and then free these 6 pages. Therefore, we need to reserve at least 2 > > pages as vmemmap areas. > > > > When a hugetlbpage is freed to the buddy system, we should allocate six > > pages for vmemmap pages and restore the previous mapping relationship. > > > > If we uses the 1G hugetlbpage, we can save 4088 pages(There are 4096 pages > > for > > struct page structures, we reserve 2 pages for vmemmap and 8 pages for page > > tables. So we can save 4088 pages). This is a very substantial gain. On our > > server, run some SPDK/QEMU applications which will use 1024GB hugetlbpage. > > With this feature enabled, we can save ~16GB(1G hugepage)/~11GB(2MB > > hugepage) > > Hi Muchun, > > Do we really save 11GB for 2MB hugepage? > How much do we save if we only get one 2MB hugetlb from one 128MB mem_section? > It seems we need to get at least one page for the PTEs since we are splitting PMD of > vmemmap into PTE? There are 524288(1024GB/2MB) 2MB HugeTLB pages. We can save 6 pages for each 2MB HugeTLB page. So we can save 3145728 pages. But we need to split PMD page table for every one 128MB mem_section and every section need one page as PTE page table. So we need 8192(1024GB/128MB) pages as PTE page tables. Finally, we can save 3137536(3145728-8192) pages which is 11.97GB. Thanks Barry. > > > memory. > > > > Because there are vmemmap page tables reconstruction on the > > freeing/allocating > > path, it increases some overhead. Here are some overhead analysis. > > > > 1) Allocating 10240 2MB hugetlb pages. > > > > a) With this patch series applied: > > # time echo 10240 > /proc/sys/vm/nr_hugepages > > > > real 0m0.166s > > user 0m0.000s > > sys 0m0.166s > > > > # bpftrace -e 'kprobe:alloc_fresh_huge_page { @start[tid] = nsecs; } > > kretprobe:alloc_fresh_huge_page /@start[tid]/ { @latency = hist(nsecs - > > @start[tid]); delete(@start[tid]); }' > > Attaching 2 probes... > > > > @latency: > > [8K, 16K) 8360 > > |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ > > @@@@@@@@@@@@@@@@| > > [16K, 32K) 1868 |@@@@@@@@@@@ > > | > > [32K, 64K) 10 | > > | > > [64K, 128K) 2 | > > | > > > > b) Without this patch series: > > # time echo 10240 > /proc/sys/vm/nr_hugepages > > > > real 0m0.066s > > user 0m0.000s > > sys 0m0.066s > > > > # bpftrace -e 'kprobe:alloc_fresh_huge_page { @start[tid] = nsecs; } > > kretprobe:alloc_fresh_huge_page /@start[tid]/ { @latency = hist(nsecs - > > @start[tid]); delete(@start[tid]); }' > > Attaching 2 probes... > > > > @latency: > > [4K, 8K) 10176 > > |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ > > @@@@@@@@@@@@@@@@| > > [8K, 16K) 62 | > > | > > [16K, 32K) 2 | > > | > > > > Summarize: this feature is about ~2x slower than before. > > > > 2) Freeing 10240 @MB hugetlb pages. > > > > a) With this patch series applied: > > # time echo 0 > /proc/sys/vm/nr_hugepages > > > > real 0m0.004s > > user 0m0.000s > > sys 0m0.002s > > > > Something is wrong here, it is faster than the case without this patchset: > 0.004s vs. 0m0.077s Yeah, it is faster. And Why the 'real' time of patched is smaller than before? Because in this patch series, the freeing HugeTLB is asynchronous(through worker). > > > # bpftrace -e 'kprobe:__free_hugepage { @start[tid] = nsecs; } > > kretprobe:__free_hugepage /@start[tid]/ { @latency = hist(nsecs - @start[tid]); > > delete(@start[tid]); }' > > Attaching 2 probes... > > > > @latency: > > [16K, 32K) 10240 > > |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ > > @@@@@@@@@@@@@@@@| > > > > b) Without this patch series: > > # time echo 0 > /proc/sys/vm/nr_hugepages > > > > real 0m0.077s > > user 0m0.001s > > sys 0m0.075s > > > > # bpftrace -e 'kprobe:__free_hugepage { @start[tid] = nsecs; } > > kretprobe:__free_hugepage /@start[tid]/ { @latency = hist(nsecs - @start[tid]); > > delete(@start[tid]); }' > > Attaching 2 probes... > > > > @latency: > > [4K, 8K) 9950 > > |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ > > @@@@@@@@@@@@@@@@| > > [8K, 16K) 287 |@ > > | > > [16K, 32K) 3 | > > | > > > > Summarize: The overhead of __free_hugepage is about ~2-4x slower than > > before. > > But according to the allocation test above, I think that here is > > also ~2x slower than before. > > > > But why the 'real' time of patched is smaller than before? > > Because > > In this patch series, the freeing hugetlb is asynchronous(through > > kwoker). > > > > Although the overhead has increased, the overhead is not significant. Like MIke > > said, "However, remember that the majority of use cases create hugetlb pages > > at > > or shortly after boot time and add them to the pool. So, additional overhead is > > at pool creation time. There is no change to 'normal run time' operations of > > getting a page from or returning a page to the pool (think page fault/unmap)". > > > > It seems it is true. At runtime, people normally don't change hugetlb. > > > changelog in v4: > > 1. Move all the vmemmap functions to hugetlb_vmemmap.c. > > 2. Make the CONFIG_HUGETLB_PAGE_FREE_VMEMMAP default to y, if we > > want to > > disable this feature, we should disable it by a boot/kernel command line. > > 3. Remove vmemmap_pgtable_{init, deposit, withdraw}() helper functions. > > 4. Initialize page table lock for vmemmap through core_initcall mechanism. > > > > Thanks for Mike and Oscar's suggestions. > > > > changelog in v3: > > 1. Rename some helps function name. Thanks Mike. > > 2. Rework some code. Thanks Mike and Oscar. > > 3. Remap the tail vmemmap page with PAGE_KERNEL_RO instead of > > PAGE_KERNEL. Thanks Matthew. > > 4. Add some overhead analysis in the cover letter. > > 5. Use vmemap pmd table lock instead of a hugetlb specific global lock. > > > > changelog in v2: > > 1. Fix do not call dissolve_compound_page in alloc_huge_page_vmemmap(). > > 2. Fix some typo and code style problems. > > 3. Remove unused handle_vmemmap_fault(). > > 4. Merge some commits to one commit suggested by Mike. > > > > Muchun Song (21): > > mm/memory_hotplug: Move bootmem info registration API to > > bootmem_info.c > > mm/memory_hotplug: Move {get,put}_page_bootmem() to bootmem_info.c > > mm/hugetlb: Introduce a new config HUGETLB_PAGE_FREE_VMEMMAP > > mm/hugetlb: Introduce nr_free_vmemmap_pages in the struct hstate > > mm/hugetlb: Introduce pgtable allocation/freeing helpers > > mm/bootmem_info: Introduce {free,prepare}_vmemmap_page() > > mm/bootmem_info: Combine bootmem info and type into page->freelist > > mm/hugetlb: Initialize page table lock for vmemmap > > mm/hugetlb: Free the vmemmap pages associated with each hugetlb page > > mm/hugetlb: Defer freeing of hugetlb pages > > mm/hugetlb: Allocate the vmemmap pages associated with each hugetlb > > page > > mm/hugetlb: Introduce remap_huge_page_pmd_vmemmap helper > > mm/hugetlb: Use PG_slab to indicate split pmd > > mm/hugetlb: Support freeing vmemmap pages of gigantic page > > mm/hugetlb: Set the PageHWPoison to the raw error page > > mm/hugetlb: Flush work when dissolving hugetlb page > > mm/hugetlb: Add a kernel parameter hugetlb_free_vmemmap > > mm/hugetlb: Merge pte to huge pmd only for gigantic page > > mm/hugetlb: Gather discrete indexes of tail page > > mm/hugetlb: Add BUILD_BUG_ON to catch invalid usage of tail struct > > page > > mm/hugetlb: Disable freeing vmemmap if struct page size is not power > > of two > > > > Documentation/admin-guide/kernel-parameters.txt | 9 + > > Documentation/admin-guide/mm/hugetlbpage.rst | 3 + > > arch/x86/include/asm/hugetlb.h | 17 + > > arch/x86/include/asm/pgtable_64_types.h | 8 + > > arch/x86/mm/init_64.c | 7 +- > > fs/Kconfig | 14 + > > include/linux/bootmem_info.h | 78 +++ > > include/linux/hugetlb.h | 19 + > > include/linux/hugetlb_cgroup.h | 15 +- > > include/linux/memory_hotplug.h | 27 - > > mm/Makefile | 2 + > > mm/bootmem_info.c | 124 ++++ > > mm/hugetlb.c | 163 +++++- > > mm/hugetlb_vmemmap.c | 732 > > ++++++++++++++++++++++++ > > mm/hugetlb_vmemmap.h | 104 ++++ > > mm/memory_hotplug.c | 116 ---- > > mm/sparse.c | 5 +- > > 17 files changed, 1263 insertions(+), 180 deletions(-) > > create mode 100644 include/linux/bootmem_info.h > > create mode 100644 mm/bootmem_info.c > > create mode 100644 mm/hugetlb_vmemmap.c > > create mode 100644 mm/hugetlb_vmemmap.h > > > > Thanks > Barry >
> -----Original Message----- > From: Muchun Song [mailto:songmuchun@bytedance.com] > Sent: Tuesday, November 17, 2020 11:50 PM > To: Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com> > Cc: corbet@lwn.net; mike.kravetz@oracle.com; tglx@linutronix.de; > mingo@redhat.com; bp@alien8.de; x86@kernel.org; hpa@zytor.com; > dave.hansen@linux.intel.com; luto@kernel.org; peterz@infradead.org; > viro@zeniv.linux.org.uk; akpm@linux-foundation.org; paulmck@kernel.org; > mchehab+huawei@kernel.org; pawan.kumar.gupta@linux.intel.com; > rdunlap@infradead.org; oneukum@suse.com; anshuman.khandual@arm.com; > jroedel@suse.de; almasrymina@google.com; rientjes@google.com; > willy@infradead.org; osalvador@suse.de; mhocko@suse.com; > duanxiongchun@bytedance.com; linux-doc@vger.kernel.org; > linux-kernel@vger.kernel.org; linux-mm@kvack.org; > linux-fsdevel@vger.kernel.org > Subject: Re: [External] RE: [PATCH v4 00/21] Free some vmemmap pages of > hugetlb page > > On Tue, Nov 17, 2020 at 6:16 PM Song Bao Hua (Barry Song) > <song.bao.hua@hisilicon.com> wrote: > > > > > > > > > -----Original Message----- > > > From: owner-linux-mm@kvack.org [mailto:owner-linux-mm@kvack.org] On > > > Behalf Of Muchun Song > > > Sent: Saturday, November 14, 2020 12:00 AM > > > To: corbet@lwn.net; mike.kravetz@oracle.com; tglx@linutronix.de; > > > mingo@redhat.com; bp@alien8.de; x86@kernel.org; hpa@zytor.com; > > > dave.hansen@linux.intel.com; luto@kernel.org; peterz@infradead.org; > > > viro@zeniv.linux.org.uk; akpm@linux-foundation.org; paulmck@kernel.org; > > > mchehab+huawei@kernel.org; pawan.kumar.gupta@linux.intel.com; > > > rdunlap@infradead.org; oneukum@suse.com; > anshuman.khandual@arm.com; > > > jroedel@suse.de; almasrymina@google.com; rientjes@google.com; > > > willy@infradead.org; osalvador@suse.de; mhocko@suse.com > > > Cc: duanxiongchun@bytedance.com; linux-doc@vger.kernel.org; > > > linux-kernel@vger.kernel.org; linux-mm@kvack.org; > > > linux-fsdevel@vger.kernel.org; Muchun Song > <songmuchun@bytedance.com> > > > Subject: [PATCH v4 00/21] Free some vmemmap pages of hugetlb page > > > > > > Hi all, > > > > > > This patch series will free some vmemmap pages(struct page structures) > > > associated with each hugetlbpage when preallocated to save memory. > > > > > > Nowadays we track the status of physical page frames using struct page > > > structures arranged in one or more arrays. And here exists one-to-one > > > mapping between the physical page frame and the corresponding struct > page > > > structure. > > > > > > The HugeTLB support is built on top of multiple page size support that > > > is provided by most modern architectures. For example, x86 CPUs normally > > > support 4K and 2M (1G if architecturally supported) page sizes. Every > > > HugeTLB has more than one struct page structure. The 2M HugeTLB has > 512 > > > struct page structure and 1G HugeTLB has 4096 struct page structures. But > > > in the core of HugeTLB only uses the first 4 (Use of first 4 struct page > > > structures comes from HUGETLB_CGROUP_MIN_ORDER.) struct page > > > structures to > > > store metadata associated with each HugeTLB. The rest of the struct page > > > structures are usually read the compound_head field which are all the same > > > value. If we can free some struct page memory to buddy system so that we > > > can save a lot of memory. > > > > > > When the system boot up, every 2M HugeTLB has 512 struct page > structures > > > which size is 8 pages(sizeof(struct page) * 512 / PAGE_SIZE). > > > > > > hugetlbpage struct pages(8 pages) page > > > frame(8 pages) > > > +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ > > > | | | 0 | -------------> | > 0 > > > | > > > | | | 1 | -------------> | > 1 > > > | > > > | | | 2 | -------------> | > 2 > > > | > > > | | | 3 | -------------> | > 3 > > > | > > > | | | 4 | -------------> | > 4 > > > | > > > | 2M | | 5 | -------------> | > > > 5 | > > > | | | 6 | -------------> | > 6 > > > | > > > | | | 7 | -------------> | > 7 > > > | > > > | | +-----------+ > > > +-----------+ > > > | | > > > | | > > > +-----------+ > > > > > > > > > When a hugetlbpage is preallocated, we can change the mapping from > above > > > to > > > bellow. > > > > > > hugetlbpage struct pages(8 pages) page > > > frame(8 pages) > > > +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ > > > | | | 0 | -------------> | > 0 > > > | > > > | | | 1 | -------------> | > 1 > > > | > > > | | | 2 | -------------> > > > +-----------+ > > > | | | 3 | -----------------^ ^ > ^ ^ > > > ^ > > > | | | 4 | -------------------+ > | | > > > | > > > | 2M | | 5 | > ---------------------+ | > > > | > > > | | | 6 | > -----------------------+ | > > > | | | 7 | > -------------------------+ > > > | | +-----------+ > > > | | > > > | | > > > +-----------+ > > > > > > For tail pages, the value of compound_head is the same. So we can reuse > > > first page of tail page structs. We map the virtual addresses of the > > > remaining 6 pages of tail page structs to the first tail page struct, > > > and then free these 6 pages. Therefore, we need to reserve at least 2 > > > pages as vmemmap areas. > > > > > > When a hugetlbpage is freed to the buddy system, we should allocate six > > > pages for vmemmap pages and restore the previous mapping relationship. > > > > > > If we uses the 1G hugetlbpage, we can save 4088 pages(There are 4096 > pages > > > for > > > struct page structures, we reserve 2 pages for vmemmap and 8 pages for > page > > > tables. So we can save 4088 pages). This is a very substantial gain. On our > > > server, run some SPDK/QEMU applications which will use 1024GB > hugetlbpage. > > > With this feature enabled, we can save ~16GB(1G hugepage)/~11GB(2MB > > > hugepage) > > > > Hi Muchun, > > > > Do we really save 11GB for 2MB hugepage? > > How much do we save if we only get one 2MB hugetlb from one 128MB > mem_section? > > It seems we need to get at least one page for the PTEs since we are splitting > PMD of > > vmemmap into PTE? > > There are 524288(1024GB/2MB) 2MB HugeTLB pages. We can save 6 pages for > each > 2MB HugeTLB page. So we can save 3145728 pages. But we need to split PMD > page > table for every one 128MB mem_section and every section need one page > as PTE page > table. So we need 8192(1024GB/128MB) pages as PTE page tables. > Finally, we can save > 3137536(3145728-8192) pages which is 11.97GB. The worst case I can see is that: if we get 100 hugetlb with 2MB size, but the 100 hugetlb comes from different mem_section, we won't save 11.97GB. we only save 5/8 * 16GB=10GB. Anyway, it seems 11GB is in the middle of 10GB and 11.97GB, so sounds sensible :-) ideally, we should be able to free PageTail if we change struct page in some way. Then we will save much more for 2MB hugetlb. but it seems it is not easy. Thanks Barry
On Tue, Nov 17, 2020 at 7:08 PM Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com> wrote: > > > > > -----Original Message----- > > From: Muchun Song [mailto:songmuchun@bytedance.com] > > Sent: Tuesday, November 17, 2020 11:50 PM > > To: Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com> > > Cc: corbet@lwn.net; mike.kravetz@oracle.com; tglx@linutronix.de; > > mingo@redhat.com; bp@alien8.de; x86@kernel.org; hpa@zytor.com; > > dave.hansen@linux.intel.com; luto@kernel.org; peterz@infradead.org; > > viro@zeniv.linux.org.uk; akpm@linux-foundation.org; paulmck@kernel.org; > > mchehab+huawei@kernel.org; pawan.kumar.gupta@linux.intel.com; > > rdunlap@infradead.org; oneukum@suse.com; anshuman.khandual@arm.com; > > jroedel@suse.de; almasrymina@google.com; rientjes@google.com; > > willy@infradead.org; osalvador@suse.de; mhocko@suse.com; > > duanxiongchun@bytedance.com; linux-doc@vger.kernel.org; > > linux-kernel@vger.kernel.org; linux-mm@kvack.org; > > linux-fsdevel@vger.kernel.org > > Subject: Re: [External] RE: [PATCH v4 00/21] Free some vmemmap pages of > > hugetlb page > > > > On Tue, Nov 17, 2020 at 6:16 PM Song Bao Hua (Barry Song) > > <song.bao.hua@hisilicon.com> wrote: > > > > > > > > > > > > > -----Original Message----- > > > > From: owner-linux-mm@kvack.org [mailto:owner-linux-mm@kvack.org] On > > > > Behalf Of Muchun Song > > > > Sent: Saturday, November 14, 2020 12:00 AM > > > > To: corbet@lwn.net; mike.kravetz@oracle.com; tglx@linutronix.de; > > > > mingo@redhat.com; bp@alien8.de; x86@kernel.org; hpa@zytor.com; > > > > dave.hansen@linux.intel.com; luto@kernel.org; peterz@infradead.org; > > > > viro@zeniv.linux.org.uk; akpm@linux-foundation.org; paulmck@kernel.org; > > > > mchehab+huawei@kernel.org; pawan.kumar.gupta@linux.intel.com; > > > > rdunlap@infradead.org; oneukum@suse.com; > > anshuman.khandual@arm.com; > > > > jroedel@suse.de; almasrymina@google.com; rientjes@google.com; > > > > willy@infradead.org; osalvador@suse.de; mhocko@suse.com > > > > Cc: duanxiongchun@bytedance.com; linux-doc@vger.kernel.org; > > > > linux-kernel@vger.kernel.org; linux-mm@kvack.org; > > > > linux-fsdevel@vger.kernel.org; Muchun Song > > <songmuchun@bytedance.com> > > > > Subject: [PATCH v4 00/21] Free some vmemmap pages of hugetlb page > > > > > > > > Hi all, > > > > > > > > This patch series will free some vmemmap pages(struct page structures) > > > > associated with each hugetlbpage when preallocated to save memory. > > > > > > > > Nowadays we track the status of physical page frames using struct page > > > > structures arranged in one or more arrays. And here exists one-to-one > > > > mapping between the physical page frame and the corresponding struct > > page > > > > structure. > > > > > > > > The HugeTLB support is built on top of multiple page size support that > > > > is provided by most modern architectures. For example, x86 CPUs normally > > > > support 4K and 2M (1G if architecturally supported) page sizes. Every > > > > HugeTLB has more than one struct page structure. The 2M HugeTLB has > > 512 > > > > struct page structure and 1G HugeTLB has 4096 struct page structures. But > > > > in the core of HugeTLB only uses the first 4 (Use of first 4 struct page > > > > structures comes from HUGETLB_CGROUP_MIN_ORDER.) struct page > > > > structures to > > > > store metadata associated with each HugeTLB. The rest of the struct page > > > > structures are usually read the compound_head field which are all the same > > > > value. If we can free some struct page memory to buddy system so that we > > > > can save a lot of memory. > > > > > > > > When the system boot up, every 2M HugeTLB has 512 struct page > > structures > > > > which size is 8 pages(sizeof(struct page) * 512 / PAGE_SIZE). > > > > > > > > hugetlbpage struct pages(8 pages) page > > > > frame(8 pages) > > > > +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ > > > > | | | 0 | -------------> | > > 0 > > > > | > > > > | | | 1 | -------------> | > > 1 > > > > | > > > > | | | 2 | -------------> | > > 2 > > > > | > > > > | | | 3 | -------------> | > > 3 > > > > | > > > > | | | 4 | -------------> | > > 4 > > > > | > > > > | 2M | | 5 | -------------> | > > > > 5 | > > > > | | | 6 | -------------> | > > 6 > > > > | > > > > | | | 7 | -------------> | > > 7 > > > > | > > > > | | +-----------+ > > > > +-----------+ > > > > | | > > > > | | > > > > +-----------+ > > > > > > > > > > > > When a hugetlbpage is preallocated, we can change the mapping from > > above > > > > to > > > > bellow. > > > > > > > > hugetlbpage struct pages(8 pages) page > > > > frame(8 pages) > > > > +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ > > > > | | | 0 | -------------> | > > 0 > > > > | > > > > | | | 1 | -------------> | > > 1 > > > > | > > > > | | | 2 | -------------> > > > > +-----------+ > > > > | | | 3 | -----------------^ ^ > > ^ ^ > > > > ^ > > > > | | | 4 | -------------------+ > > | | > > > > | > > > > | 2M | | 5 | > > ---------------------+ | > > > > | > > > > | | | 6 | > > -----------------------+ | > > > > | | | 7 | > > -------------------------+ > > > > | | +-----------+ > > > > | | > > > > | | > > > > +-----------+ > > > > > > > > For tail pages, the value of compound_head is the same. So we can reuse > > > > first page of tail page structs. We map the virtual addresses of the > > > > remaining 6 pages of tail page structs to the first tail page struct, > > > > and then free these 6 pages. Therefore, we need to reserve at least 2 > > > > pages as vmemmap areas. > > > > > > > > When a hugetlbpage is freed to the buddy system, we should allocate six > > > > pages for vmemmap pages and restore the previous mapping relationship. > > > > > > > > If we uses the 1G hugetlbpage, we can save 4088 pages(There are 4096 > > pages > > > > for > > > > struct page structures, we reserve 2 pages for vmemmap and 8 pages for > > page > > > > tables. So we can save 4088 pages). This is a very substantial gain. On our > > > > server, run some SPDK/QEMU applications which will use 1024GB > > hugetlbpage. > > > > With this feature enabled, we can save ~16GB(1G hugepage)/~11GB(2MB > > > > hugepage) > > > > > > Hi Muchun, > > > > > > Do we really save 11GB for 2MB hugepage? > > > How much do we save if we only get one 2MB hugetlb from one 128MB > > mem_section? > > > It seems we need to get at least one page for the PTEs since we are splitting > > PMD of > > > vmemmap into PTE? > > > > There are 524288(1024GB/2MB) 2MB HugeTLB pages. We can save 6 pages for > > each > > 2MB HugeTLB page. So we can save 3145728 pages. But we need to split PMD > > page > > table for every one 128MB mem_section and every section need one page > > as PTE page > > table. So we need 8192(1024GB/128MB) pages as PTE page tables. > > Finally, we can save > > 3137536(3145728-8192) pages which is 11.97GB. > > The worst case I can see is that: > if we get 100 hugetlb with 2MB size, but the 100 hugetlb comes from different > mem_section, we won't save 11.97GB. we only save 5/8 * 16GB=10GB. > > Anyway, it seems 11GB is in the middle of 10GB and 11.97GB, > so sounds sensible :-) > > ideally, we should be able to free PageTail if we change struct page in some way. > Then we will save much more for 2MB hugetlb. but it seems it is not easy. Now for the 2MB HugrTLB page, we only free 6 vmemmap pages. But your words woke me up. Maybe we really can free 7 vmemmap pages. In this case, we can see 8 of the 512 struct page structures has beed set PG_head flag. If we can adjust compound_head() slightly and make compound_head() return the real head struct page when the parameter is the tail struct page but with PG_head flag set. I will start an investigation and a test. Thanks. > > Thanks > Barry
On Wed, Nov 18, 2020 at 12:29:07AM +0800, Muchun Song wrote: > > ideally, we should be able to free PageTail if we change struct page in some way. > > Then we will save much more for 2MB hugetlb. but it seems it is not easy. > > Now for the 2MB HugrTLB page, we only free 6 vmemmap pages. > But your words woke me up. Maybe we really can free 7 vmemmap > pages. In this case, we can see 8 of the 512 struct page structures > has beed set PG_head flag. If we can adjust compound_head() > slightly and make compound_head() return the real head struct > page when the parameter is the tail struct page but with PG_head > flag set. I will start an investigation and a test. What are you thinking? static inline struct page *compound_head(struct page *page) { unsigned long head = READ_ONCE(page->compound_head); if (unlikely(head & 1)) return (struct page *) (head - 1); + if (unlikely(page->flags & PG_head)) + return (struct page *)(page[1]->compound_head - 1) return page; } ... because if it's that, there are code paths which also just test PageHead, and so we'd actually need to change PageHead to be something like: static inline bool PageHead(struct page *page) { return (page->flags & PG_head) && (page[1]->compound_head == (unsigned long)page + 1); } I'm not sure if that's worth doing -- there may be other things I haven't thought of.
On 2020-11-17 17:29, Muchun Song wrote: > Now for the 2MB HugrTLB page, we only free 6 vmemmap pages. > But your words woke me up. Maybe we really can free 7 vmemmap > pages. In this case, we can see 8 of the 512 struct page structures > has beed set PG_head flag. If we can adjust compound_head() > slightly and make compound_head() return the real head struct > page when the parameter is the tail struct page but with PG_head > flag set. I will start an investigation and a test. I would not overcomplicate things at this stage, but rather keep it simple as the code is already tricky enough(without counting the LOC thatvit adds). We can always build on top later on in order to improve things.
On Wed, Nov 18, 2020 at 3:22 AM Matthew Wilcox <willy@infradead.org> wrote: > > On Wed, Nov 18, 2020 at 12:29:07AM +0800, Muchun Song wrote: > > > ideally, we should be able to free PageTail if we change struct page in some way. > > > Then we will save much more for 2MB hugetlb. but it seems it is not easy. > > > > Now for the 2MB HugrTLB page, we only free 6 vmemmap pages. > > But your words woke me up. Maybe we really can free 7 vmemmap > > pages. In this case, we can see 8 of the 512 struct page structures > > has beed set PG_head flag. If we can adjust compound_head() > > slightly and make compound_head() return the real head struct > > page when the parameter is the tail struct page but with PG_head > > flag set. I will start an investigation and a test. > > What are you thinking? > > static inline struct page *compound_head(struct page *page) > { > unsigned long head = READ_ONCE(page->compound_head); > > if (unlikely(head & 1)) > return (struct page *) (head - 1); > + if (unlikely(page->flags & PG_head)) > + return (struct page *)(page[1]->compound_head - 1) Yeah, I think so too. Maybe adding an align check is better. + if ((test_bit(PG_head, &page->flags) && + IS_ALIGNED((unsigned long)page, PAGE_SIZE)) > return page; > } > > ... because if it's that, there are code paths which also just test > PageHead, and so we'd actually need to change PageHead to be something > like: Yeah, I also think that rework compound_head() and PageHead() is enough. Thanks. > > static inline bool PageHead(struct page *page) > { > return (page->flags & PG_head) && > (page[1]->compound_head == (unsigned long)page + 1); > } > > I'm not sure if that's worth doing -- there may be other things I > haven't thought of.
On Wed, Nov 18, 2020 at 3:45 AM Oscar Salvador <osalvador@suse.de> wrote: > > On 2020-11-17 17:29, Muchun Song wrote: > > Now for the 2MB HugrTLB page, we only free 6 vmemmap pages. > > But your words woke me up. Maybe we really can free 7 vmemmap > > pages. In this case, we can see 8 of the 512 struct page structures > > has beed set PG_head flag. If we can adjust compound_head() > > slightly and make compound_head() return the real head struct > > page when the parameter is the tail struct page but with PG_head > > flag set. I will start an investigation and a test. > > I would not overcomplicate things at this stage, but rather keep it > simple as the code is already tricky enough(without counting the LOC > thatvit adds). > We can always build on top later on in order to improve things. I think that this improvement can be a separate patch. In this case, it can make the code evolution route clearer. Thanks. > > -- > Oscar Salvador > SUSE L3
> -----Original Message----- > From: Oscar Salvador [mailto:osalvador@suse.de] > Sent: Wednesday, November 18, 2020 8:45 AM > To: Muchun Song <songmuchun@bytedance.com> > Cc: Song Bao Hua (Barry Song) <song.bao.hua@hisilicon.com>; > corbet@lwn.net; mike.kravetz@oracle.com; tglx@linutronix.de; > mingo@redhat.com; bp@alien8.de; x86@kernel.org; hpa@zytor.com; > dave.hansen@linux.intel.com; luto@kernel.org; peterz@infradead.org; > viro@zeniv.linux.org.uk; akpm@linux-foundation.org; paulmck@kernel.org; > mchehab+huawei@kernel.org; pawan.kumar.gupta@linux.intel.com; > rdunlap@infradead.org; oneukum@suse.com; anshuman.khandual@arm.com; > jroedel@suse.de; almasrymina@google.com; rientjes@google.com; > willy@infradead.org; mhocko@suse.com; duanxiongchun@bytedance.com; > linux-doc@vger.kernel.org; linux-kernel@vger.kernel.org; linux-mm@kvack.org; > linux-fsdevel@vger.kernel.org > Subject: Re: [External] RE: [PATCH v4 00/21] Free some vmemmap pages of > hugetlb page > > On 2020-11-17 17:29, Muchun Song wrote: > > Now for the 2MB HugrTLB page, we only free 6 vmemmap pages. > > But your words woke me up. Maybe we really can free 7 vmemmap > > pages. In this case, we can see 8 of the 512 struct page structures > > has beed set PG_head flag. If we can adjust compound_head() > > slightly and make compound_head() return the real head struct > > page when the parameter is the tail struct page but with PG_head > > flag set. I will start an investigation and a test. > > I would not overcomplicate things at this stage, but rather keep it > simple as the code is already tricky enough(without counting the LOC > thatvit adds). > We can always build on top later on in order to improve things. Yep. I am not expecting freeing tail page to be done at this stage. This could be something in the todo list after the first patchset is solid. > > -- > Oscar Salvador > SUSE L3 Thanks Barry
Hi all, This patch series will free some vmemmap pages(struct page structures) associated with each hugetlbpage when preallocated to save memory. Nowadays we track the status of physical page frames using struct page structures arranged in one or more arrays. And here exists one-to-one mapping between the physical page frame and the corresponding struct page structure. The HugeTLB support is built on top of multiple page size support that is provided by most modern architectures. For example, x86 CPUs normally support 4K and 2M (1G if architecturally supported) page sizes. Every HugeTLB has more than one struct page structure. The 2M HugeTLB has 512 struct page structure and 1G HugeTLB has 4096 struct page structures. But in the core of HugeTLB only uses the first 4 (Use of first 4 struct page structures comes from HUGETLB_CGROUP_MIN_ORDER.) struct page structures to store metadata associated with each HugeTLB. The rest of the struct page structures are usually read the compound_head field which are all the same value. If we can free some struct page memory to buddy system so that we can save a lot of memory. When the system boot up, every 2M HugeTLB has 512 struct page structures which size is 8 pages(sizeof(struct page) * 512 / PAGE_SIZE). hugetlbpage struct pages(8 pages) page frame(8 pages) +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ | | | 0 | -------------> | 0 | | | | 1 | -------------> | 1 | | | | 2 | -------------> | 2 | | | | 3 | -------------> | 3 | | | | 4 | -------------> | 4 | | 2M | | 5 | -------------> | 5 | | | | 6 | -------------> | 6 | | | | 7 | -------------> | 7 | | | +-----------+ +-----------+ | | | | +-----------+ When a hugetlbpage is preallocated, we can change the mapping from above to bellow. hugetlbpage struct pages(8 pages) page frame(8 pages) +-----------+ ---virt_to_page---> +-----------+ mapping to +-----------+ | | | 0 | -------------> | 0 | | | | 1 | -------------> | 1 | | | | 2 | -------------> +-----------+ | | | 3 | -----------------^ ^ ^ ^ ^ | | | 4 | -------------------+ | | | | 2M | | 5 | ---------------------+ | | | | | 6 | -----------------------+ | | | | 7 | -------------------------+ | | +-----------+ | | | | +-----------+ For tail pages, the value of compound_head is the same. So we can reuse first page of tail page structs. We map the virtual addresses of the remaining 6 pages of tail page structs to the first tail page struct, and then free these 6 pages. Therefore, we need to reserve at least 2 pages as vmemmap areas. When a hugetlbpage is freed to the buddy system, we should allocate six pages for vmemmap pages and restore the previous mapping relationship. If we uses the 1G hugetlbpage, we can save 4088 pages(There are 4096 pages for struct page structures, we reserve 2 pages for vmemmap and 8 pages for page tables. So we can save 4088 pages). This is a very substantial gain. On our server, run some SPDK/QEMU applications which will use 1024GB hugetlbpage. With this feature enabled, we can save ~16GB(1G hugepage)/~11GB(2MB hugepage) memory. Because there are vmemmap page tables reconstruction on the freeing/allocating path, it increases some overhead. Here are some overhead analysis. 1) Allocating 10240 2MB hugetlb pages. a) With this patch series applied: # time echo 10240 > /proc/sys/vm/nr_hugepages real 0m0.166s user 0m0.000s sys 0m0.166s # bpftrace -e 'kprobe:alloc_fresh_huge_page { @start[tid] = nsecs; } kretprobe:alloc_fresh_huge_page /@start[tid]/ { @latency = hist(nsecs - @start[tid]); delete(@start[tid]); }' Attaching 2 probes... @latency: [8K, 16K) 8360 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@| [16K, 32K) 1868 |@@@@@@@@@@@ | [32K, 64K) 10 | | [64K, 128K) 2 | | b) Without this patch series: # time echo 10240 > /proc/sys/vm/nr_hugepages real 0m0.066s user 0m0.000s sys 0m0.066s # bpftrace -e 'kprobe:alloc_fresh_huge_page { @start[tid] = nsecs; } kretprobe:alloc_fresh_huge_page /@start[tid]/ { @latency = hist(nsecs - @start[tid]); delete(@start[tid]); }' Attaching 2 probes... @latency: [4K, 8K) 10176 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@| [8K, 16K) 62 | | [16K, 32K) 2 | | Summarize: this feature is about ~2x slower than before. 2) Freeing 10240 @MB hugetlb pages. a) With this patch series applied: # time echo 0 > /proc/sys/vm/nr_hugepages real 0m0.004s user 0m0.000s sys 0m0.002s # bpftrace -e 'kprobe:__free_hugepage { @start[tid] = nsecs; } kretprobe:__free_hugepage /@start[tid]/ { @latency = hist(nsecs - @start[tid]); delete(@start[tid]); }' Attaching 2 probes... @latency: [16K, 32K) 10240 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@| b) Without this patch series: # time echo 0 > /proc/sys/vm/nr_hugepages real 0m0.077s user 0m0.001s sys 0m0.075s # bpftrace -e 'kprobe:__free_hugepage { @start[tid] = nsecs; } kretprobe:__free_hugepage /@start[tid]/ { @latency = hist(nsecs - @start[tid]); delete(@start[tid]); }' Attaching 2 probes... @latency: [4K, 8K) 9950 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@| [8K, 16K) 287 |@ | [16K, 32K) 3 | | Summarize: The overhead of __free_hugepage is about ~2-4x slower than before. But according to the allocation test above, I think that here is also ~2x slower than before. But why the 'real' time of patched is smaller than before? Because In this patch series, the freeing hugetlb is asynchronous(through kwoker). Although the overhead has increased, the overhead is not significant. Like MIke said, "However, remember that the majority of use cases create hugetlb pages at or shortly after boot time and add them to the pool. So, additional overhead is at pool creation time. There is no change to 'normal run time' operations of getting a page from or returning a page to the pool (think page fault/unmap)". changelog in v4: 1. Move all the vmemmap functions to hugetlb_vmemmap.c. 2. Make the CONFIG_HUGETLB_PAGE_FREE_VMEMMAP default to y, if we want to disable this feature, we should disable it by a boot/kernel command line. 3. Remove vmemmap_pgtable_{init, deposit, withdraw}() helper functions. 4. Initialize page table lock for vmemmap through core_initcall mechanism. Thanks for Mike and Oscar's suggestions. changelog in v3: 1. Rename some helps function name. Thanks Mike. 2. Rework some code. Thanks Mike and Oscar. 3. Remap the tail vmemmap page with PAGE_KERNEL_RO instead of PAGE_KERNEL. Thanks Matthew. 4. Add some overhead analysis in the cover letter. 5. Use vmemap pmd table lock instead of a hugetlb specific global lock. changelog in v2: 1. Fix do not call dissolve_compound_page in alloc_huge_page_vmemmap(). 2. Fix some typo and code style problems. 3. Remove unused handle_vmemmap_fault(). 4. Merge some commits to one commit suggested by Mike. Muchun Song (21): mm/memory_hotplug: Move bootmem info registration API to bootmem_info.c mm/memory_hotplug: Move {get,put}_page_bootmem() to bootmem_info.c mm/hugetlb: Introduce a new config HUGETLB_PAGE_FREE_VMEMMAP mm/hugetlb: Introduce nr_free_vmemmap_pages in the struct hstate mm/hugetlb: Introduce pgtable allocation/freeing helpers mm/bootmem_info: Introduce {free,prepare}_vmemmap_page() mm/bootmem_info: Combine bootmem info and type into page->freelist mm/hugetlb: Initialize page table lock for vmemmap mm/hugetlb: Free the vmemmap pages associated with each hugetlb page mm/hugetlb: Defer freeing of hugetlb pages mm/hugetlb: Allocate the vmemmap pages associated with each hugetlb page mm/hugetlb: Introduce remap_huge_page_pmd_vmemmap helper mm/hugetlb: Use PG_slab to indicate split pmd mm/hugetlb: Support freeing vmemmap pages of gigantic page mm/hugetlb: Set the PageHWPoison to the raw error page mm/hugetlb: Flush work when dissolving hugetlb page mm/hugetlb: Add a kernel parameter hugetlb_free_vmemmap mm/hugetlb: Merge pte to huge pmd only for gigantic page mm/hugetlb: Gather discrete indexes of tail page mm/hugetlb: Add BUILD_BUG_ON to catch invalid usage of tail struct page mm/hugetlb: Disable freeing vmemmap if struct page size is not power of two Documentation/admin-guide/kernel-parameters.txt | 9 + Documentation/admin-guide/mm/hugetlbpage.rst | 3 + arch/x86/include/asm/hugetlb.h | 17 + arch/x86/include/asm/pgtable_64_types.h | 8 + arch/x86/mm/init_64.c | 7 +- fs/Kconfig | 14 + include/linux/bootmem_info.h | 78 +++ include/linux/hugetlb.h | 19 + include/linux/hugetlb_cgroup.h | 15 +- include/linux/memory_hotplug.h | 27 - mm/Makefile | 2 + mm/bootmem_info.c | 124 ++++ mm/hugetlb.c | 163 +++++- mm/hugetlb_vmemmap.c | 732 ++++++++++++++++++++++++ mm/hugetlb_vmemmap.h | 104 ++++ mm/memory_hotplug.c | 116 ---- mm/sparse.c | 5 +- 17 files changed, 1263 insertions(+), 180 deletions(-) create mode 100644 include/linux/bootmem_info.h create mode 100644 mm/bootmem_info.c create mode 100644 mm/hugetlb_vmemmap.c create mode 100644 mm/hugetlb_vmemmap.h