| Message ID | cover.1718090413.git.baolin.wang@linux.alibaba.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | add mTHP support for anonymous shmem | expand |
On Tue, Jun 11, 2024 at 06:11:04PM +0800, Baolin Wang wrote: > Anonymous pages have already been supported for multi-size (mTHP) allocation > through commit 19eaf44954df, that can allow THP to be configured through the > sysfs interface located at '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'. > > However, the anonymous shmem will ignore the anonymous mTHP rule configured > through the sysfs interface, and can only use the PMD-mapped THP, that is not > reasonable. Many implement anonymous page sharing through mmap(MAP_SHARED | > MAP_ANONYMOUS), especially in database usage scenarios, therefore, users expect > to apply an unified mTHP strategy for anonymous pages, also including the > anonymous shared pages, in order to enjoy the benefits of mTHP. For example, > lower latency than PMD-mapped THP, smaller memory bloat than PMD-mapped THP, > contiguous PTEs on ARM architecture to reduce TLB miss etc. OK, this makes sense. > As discussed in the bi-weekly MM meeting[1], the mTHP controls should control > all of shmem, not only anonymous shmem, but support will be added iteratively. > Therefore, this patch set starts with support for anonymous shmem. But then this doesn't. You say first that users want the same controls to control all anonymous memory, then you introduce a completely separate set of controls for shared anonymous memory. shmem has two uses: - MAP_ANONYMOUS | MAP_SHARED (this patch set) - tmpfs For the second use case we don't want controls *at all*, we want the same heiristics used for all other filesystems to apply to tmpfs. There's no reason to have separate controls for choosing folio size in shmem. > The primary strategy is similar to supporting anonymous mTHP. Introduce > a new interface '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled', > which can have almost the same values as the top-level > '/sys/kernel/mm/transparent_hugepage/shmem_enabled', with adding a new > additional "inherit" option and dropping the testing options 'force' and > 'deny'. By default all sizes will be set to "never" except PMD size, which > is set to "inherit". This ensures backward compatibility with the anonymous > shmem enabled of the top level, meanwhile also allows independent control of > anonymous shmem enabled for each mTHP.
> shmem has two uses: > > - MAP_ANONYMOUS | MAP_SHARED (this patch set) > - tmpfs > > For the second use case we don't want controls *at all*, we want the > same heiristics used for all other filesystems to apply to tmpfs. As discussed in the MM meeting, Hugh had a different opinion on that.
On 04.07.24 21:03, David Hildenbrand wrote: >> shmem has two uses: >> >> - MAP_ANONYMOUS | MAP_SHARED (this patch set) >> - tmpfs >> >> For the second use case we don't want controls *at all*, we want the >> same heiristics used for all other filesystems to apply to tmpfs. > > As discussed in the MM meeting, Hugh had a different opinion on that. FWIW, I just recalled that I wrote a quick summary: https://lkml.kernel.org/r/f1783ff0-65bd-4b2b-8952-52b6822a0835@redhat.com I believe the meetings are recorded as well, but never looked at recordings.
On Thu, Jul 04, 2024 at 09:19:10PM +0200, David Hildenbrand wrote: > On 04.07.24 21:03, David Hildenbrand wrote: > > > shmem has two uses: > > > > > > - MAP_ANONYMOUS | MAP_SHARED (this patch set) > > > - tmpfs > > > > > > For the second use case we don't want controls *at all*, we want the > > > same heiristics used for all other filesystems to apply to tmpfs. > > > > As discussed in the MM meeting, Hugh had a different opinion on that. > > FWIW, I just recalled that I wrote a quick summary: > > https://lkml.kernel.org/r/f1783ff0-65bd-4b2b-8952-52b6822a0835@redhat.com > > I believe the meetings are recorded as well, but never looked at recordings. That's not what I understood Hugh to mean. To me, it seemed that Hugh was expressing an opinion on using shmem as shmem, not as using it as tmpfs. If I misunderstood Hugh, well, I still disagree. We should not have separate controls for this. tmpfs is just not that special.
On 2024/7/5 03:49, Matthew Wilcox wrote: > On Thu, Jul 04, 2024 at 09:19:10PM +0200, David Hildenbrand wrote: >> On 04.07.24 21:03, David Hildenbrand wrote: >>>> shmem has two uses: >>>> >>>> - MAP_ANONYMOUS | MAP_SHARED (this patch set) >>>> - tmpfs >>>> >>>> For the second use case we don't want controls *at all*, we want the >>>> same heiristics used for all other filesystems to apply to tmpfs. >>> >>> As discussed in the MM meeting, Hugh had a different opinion on that. >> >> FWIW, I just recalled that I wrote a quick summary: >> >> https://lkml.kernel.org/r/f1783ff0-65bd-4b2b-8952-52b6822a0835@redhat.com >> >> I believe the meetings are recorded as well, but never looked at recordings. > > That's not what I understood Hugh to mean. To me, it seemed that Hugh > was expressing an opinion on using shmem as shmem, not as using it as > tmpfs. > > If I misunderstood Hugh, well, I still disagree. We should not have > separate controls for this. tmpfs is just not that special. But now we already have a PMD-mapped THP control for tmpfs, and mTHP simply extends this control to per-size. IIUC, as David mentioned before, for tmpfs, mTHP should act like a huge order filter which should be respected by the expected huge orders in the write() and fallocate() paths. This would also solve the issue of allocating huge orders in writable mmap() path for tmpfs, as well as unifying the interface. Anyway, I will try to provide an RFC to discuss the mTHP for tmpfs approach.
On 05/07/2024 06:47, Baolin Wang wrote: > > > On 2024/7/5 03:49, Matthew Wilcox wrote: >> On Thu, Jul 04, 2024 at 09:19:10PM +0200, David Hildenbrand wrote: >>> On 04.07.24 21:03, David Hildenbrand wrote: >>>>> shmem has two uses: >>>>> >>>>> - MAP_ANONYMOUS | MAP_SHARED (this patch set) >>>>> - tmpfs >>>>> >>>>> For the second use case we don't want controls *at all*, we want the >>>>> same heiristics used for all other filesystems to apply to tmpfs. >>>> >>>> As discussed in the MM meeting, Hugh had a different opinion on that. >>> >>> FWIW, I just recalled that I wrote a quick summary: >>> >>> https://lkml.kernel.org/r/f1783ff0-65bd-4b2b-8952-52b6822a0835@redhat.com >>> >>> I believe the meetings are recorded as well, but never looked at recordings. >> >> That's not what I understood Hugh to mean. To me, it seemed that Hugh >> was expressing an opinion on using shmem as shmem, not as using it as >> tmpfs. >> >> If I misunderstood Hugh, well, I still disagree. We should not have >> separate controls for this. tmpfs is just not that special. I wasn't at the meeting that's being referred to, but I thought we previously agreed that tmpfs *is* special because in some configurations its not backed by swap so is locked in ram? > > But now we already have a PMD-mapped THP control for tmpfs, and mTHP simply > extends this control to per-size. > > IIUC, as David mentioned before, for tmpfs, mTHP should act like a huge order > filter which should be respected by the expected huge orders in the write() and > fallocate() paths. This would also solve the issue of allocating huge orders in > writable mmap() path for tmpfs, as well as unifying the interface. > > Anyway, I will try to provide an RFC to discuss the mTHP for tmpfs approach.
On 05.07.24 10:45, Ryan Roberts wrote: > On 05/07/2024 06:47, Baolin Wang wrote: >> >> >> On 2024/7/5 03:49, Matthew Wilcox wrote: >>> On Thu, Jul 04, 2024 at 09:19:10PM +0200, David Hildenbrand wrote: >>>> On 04.07.24 21:03, David Hildenbrand wrote: >>>>>> shmem has two uses: >>>>>> >>>>>> - MAP_ANONYMOUS | MAP_SHARED (this patch set) >>>>>> - tmpfs >>>>>> >>>>>> For the second use case we don't want controls *at all*, we want the >>>>>> same heiristics used for all other filesystems to apply to tmpfs. >>>>> >>>>> As discussed in the MM meeting, Hugh had a different opinion on that. >>>> >>>> FWIW, I just recalled that I wrote a quick summary: >>>> >>>> https://lkml.kernel.org/r/f1783ff0-65bd-4b2b-8952-52b6822a0835@redhat.com >>>> >>>> I believe the meetings are recorded as well, but never looked at recordings. >>> >>> That's not what I understood Hugh to mean. To me, it seemed that Hugh >>> was expressing an opinion on using shmem as shmem, not as using it as >>> tmpfs. >>> >>> If I misunderstood Hugh, well, I still disagree. We should not have >>> separate controls for this. tmpfs is just not that special. > > I wasn't at the meeting that's being referred to, but I thought we previously > agreed that tmpfs *is* special because in some configurations its not backed by > swap so is locked in ram? There are multiple things to that, like: * Machines only having limited/no swap configured * tmpfs can be configured to never go to swap * memfd/tmpfs files getting used purely for mmap(): there is no real difference to MAP_ANON|MAP_SHARE besides the processes we share that memory with. Especially when it comes to memory waste concerns and access behavior in some cases, tmpfs behaved much more like anonymous memory. But there are for sure other use cases where tmpfs is not that special. My opinion is that we need to let people configure orders (if you feel like it, configure all), but *select* the order to allocate based on readahead information -- in contrast to anonymous memory where we start at the highest order and don't have readahead information available. Maybe we need different "order allcoation" logic for read/write vs. fault, not sure. But I don't maintain that code, so I can only give stupid suggestions and repeat what I understood from the meeting with Hugh and Kirill :)
On 05/07/2024 09:59, David Hildenbrand wrote: > On 05.07.24 10:45, Ryan Roberts wrote: >> On 05/07/2024 06:47, Baolin Wang wrote: >>> >>> >>> On 2024/7/5 03:49, Matthew Wilcox wrote: >>>> On Thu, Jul 04, 2024 at 09:19:10PM +0200, David Hildenbrand wrote: >>>>> On 04.07.24 21:03, David Hildenbrand wrote: >>>>>>> shmem has two uses: >>>>>>> >>>>>>> - MAP_ANONYMOUS | MAP_SHARED (this patch set) >>>>>>> - tmpfs >>>>>>> >>>>>>> For the second use case we don't want controls *at all*, we want the >>>>>>> same heiristics used for all other filesystems to apply to tmpfs. >>>>>> >>>>>> As discussed in the MM meeting, Hugh had a different opinion on that. >>>>> >>>>> FWIW, I just recalled that I wrote a quick summary: >>>>> >>>>> https://lkml.kernel.org/r/f1783ff0-65bd-4b2b-8952-52b6822a0835@redhat.com >>>>> >>>>> I believe the meetings are recorded as well, but never looked at recordings. >>>> >>>> That's not what I understood Hugh to mean. To me, it seemed that Hugh >>>> was expressing an opinion on using shmem as shmem, not as using it as >>>> tmpfs. >>>> >>>> If I misunderstood Hugh, well, I still disagree. We should not have >>>> separate controls for this. tmpfs is just not that special. >> >> I wasn't at the meeting that's being referred to, but I thought we previously >> agreed that tmpfs *is* special because in some configurations its not backed by >> swap so is locked in ram? > > There are multiple things to that, like: > > * Machines only having limited/no swap configured > * tmpfs can be configured to never go to swap > * memfd/tmpfs files getting used purely for mmap(): there is no real > difference to MAP_ANON|MAP_SHARE besides the processes we share that > memory with. > > Especially when it comes to memory waste concerns and access behavior in some > cases, tmpfs behaved much more like anonymous memory. But there are for sure > other use cases where tmpfs is not that special. > > My opinion is that we need to let people configure orders (if you feel like it, > configure all), but *select* the order to allocate based on readahead > information -- in contrast to anonymous memory where we start at the highest > order and don't have readahead information available. That approach is exactly what I proposed to start playing with yesterday [1] for regular pagecache folio allocations too :) [1] https://lore.kernel.org/linux-mm/bdde4008-60db-4717-a6b5-53d77ab76bdb@arm.com/ > > Maybe we need different "order allcoation" logic for read/write vs. fault, not > sure. > > But I don't maintain that code, so I can only give stupid suggestions and repeat > what I understood from the meeting with Hugh and Kirill :) >
On 05.07.24 11:13, Ryan Roberts wrote: > On 05/07/2024 09:59, David Hildenbrand wrote: >> On 05.07.24 10:45, Ryan Roberts wrote: >>> On 05/07/2024 06:47, Baolin Wang wrote: >>>> >>>> >>>> On 2024/7/5 03:49, Matthew Wilcox wrote: >>>>> On Thu, Jul 04, 2024 at 09:19:10PM +0200, David Hildenbrand wrote: >>>>>> On 04.07.24 21:03, David Hildenbrand wrote: >>>>>>>> shmem has two uses: >>>>>>>> >>>>>>>> - MAP_ANONYMOUS | MAP_SHARED (this patch set) >>>>>>>> - tmpfs >>>>>>>> >>>>>>>> For the second use case we don't want controls *at all*, we want the >>>>>>>> same heiristics used for all other filesystems to apply to tmpfs. >>>>>>> >>>>>>> As discussed in the MM meeting, Hugh had a different opinion on that. >>>>>> >>>>>> FWIW, I just recalled that I wrote a quick summary: >>>>>> >>>>>> https://lkml.kernel.org/r/f1783ff0-65bd-4b2b-8952-52b6822a0835@redhat.com >>>>>> >>>>>> I believe the meetings are recorded as well, but never looked at recordings. >>>>> >>>>> That's not what I understood Hugh to mean. To me, it seemed that Hugh >>>>> was expressing an opinion on using shmem as shmem, not as using it as >>>>> tmpfs. >>>>> >>>>> If I misunderstood Hugh, well, I still disagree. We should not have >>>>> separate controls for this. tmpfs is just not that special. >>> >>> I wasn't at the meeting that's being referred to, but I thought we previously >>> agreed that tmpfs *is* special because in some configurations its not backed by >>> swap so is locked in ram? >> >> There are multiple things to that, like: >> >> * Machines only having limited/no swap configured >> * tmpfs can be configured to never go to swap >> * memfd/tmpfs files getting used purely for mmap(): there is no real >> difference to MAP_ANON|MAP_SHARE besides the processes we share that >> memory with. >> >> Especially when it comes to memory waste concerns and access behavior in some >> cases, tmpfs behaved much more like anonymous memory. But there are for sure >> other use cases where tmpfs is not that special. >> >> My opinion is that we need to let people configure orders (if you feel like it, >> configure all), but *select* the order to allocate based on readahead >> information -- in contrast to anonymous memory where we start at the highest >> order and don't have readahead information available. > > That approach is exactly what I proposed to start playing with yesterday [1] for > regular pagecache folio allocations too :) In German, there is this saying "zwei Dumme ein Gedanke". The official English alternative is "great minds think alike". ... well, the direct German->English translation definitely has a "German touch" to it: "two stupid ones one thought"
On 05/07/2024 10:16, David Hildenbrand wrote: > On 05.07.24 11:13, Ryan Roberts wrote: >> On 05/07/2024 09:59, David Hildenbrand wrote: >>> On 05.07.24 10:45, Ryan Roberts wrote: >>>> On 05/07/2024 06:47, Baolin Wang wrote: >>>>> >>>>> >>>>> On 2024/7/5 03:49, Matthew Wilcox wrote: >>>>>> On Thu, Jul 04, 2024 at 09:19:10PM +0200, David Hildenbrand wrote: >>>>>>> On 04.07.24 21:03, David Hildenbrand wrote: >>>>>>>>> shmem has two uses: >>>>>>>>> >>>>>>>>> - MAP_ANONYMOUS | MAP_SHARED (this patch set) >>>>>>>>> - tmpfs >>>>>>>>> >>>>>>>>> For the second use case we don't want controls *at all*, we want the >>>>>>>>> same heiristics used for all other filesystems to apply to tmpfs. >>>>>>>> >>>>>>>> As discussed in the MM meeting, Hugh had a different opinion on that. >>>>>>> >>>>>>> FWIW, I just recalled that I wrote a quick summary: >>>>>>> >>>>>>> https://lkml.kernel.org/r/f1783ff0-65bd-4b2b-8952-52b6822a0835@redhat.com >>>>>>> >>>>>>> I believe the meetings are recorded as well, but never looked at recordings. >>>>>> >>>>>> That's not what I understood Hugh to mean. To me, it seemed that Hugh >>>>>> was expressing an opinion on using shmem as shmem, not as using it as >>>>>> tmpfs. >>>>>> >>>>>> If I misunderstood Hugh, well, I still disagree. We should not have >>>>>> separate controls for this. tmpfs is just not that special. >>>> >>>> I wasn't at the meeting that's being referred to, but I thought we previously >>>> agreed that tmpfs *is* special because in some configurations its not backed by >>>> swap so is locked in ram? >>> >>> There are multiple things to that, like: >>> >>> * Machines only having limited/no swap configured >>> * tmpfs can be configured to never go to swap >>> * memfd/tmpfs files getting used purely for mmap(): there is no real >>> difference to MAP_ANON|MAP_SHARE besides the processes we share that >>> memory with. >>> >>> Especially when it comes to memory waste concerns and access behavior in some >>> cases, tmpfs behaved much more like anonymous memory. But there are for sure >>> other use cases where tmpfs is not that special. >>> >>> My opinion is that we need to let people configure orders (if you feel like it, >>> configure all), but *select* the order to allocate based on readahead >>> information -- in contrast to anonymous memory where we start at the highest >>> order and don't have readahead information available. >> >> That approach is exactly what I proposed to start playing with yesterday [1] for >> regular pagecache folio allocations too :) > > In German, there is this saying "zwei Dumme ein Gedanke". > > The official English alternative is "great minds think alike". > > ... well, the direct German->English translation definitely has a "German touch" > to it: "two stupid ones one thought" I definitely prefer the direct translation. :)
On Fri, Jul 05, 2024 at 10:59:02AM GMT, David Hildenbrand wrote: > On 05.07.24 10:45, Ryan Roberts wrote: > > On 05/07/2024 06:47, Baolin Wang wrote: > > > > > > > > > On 2024/7/5 03:49, Matthew Wilcox wrote: > > > > On Thu, Jul 04, 2024 at 09:19:10PM +0200, David Hildenbrand wrote: > > > > > On 04.07.24 21:03, David Hildenbrand wrote: > > > > > > > shmem has two uses: > > > > > > > > > > > > > > - MAP_ANONYMOUS | MAP_SHARED (this patch set) > > > > > > > - tmpfs > > > > > > > > > > > > > > For the second use case we don't want controls *at all*, we want the > > > > > > > same heiristics used for all other filesystems to apply to tmpfs. > > > > > > > > > > > > As discussed in the MM meeting, Hugh had a different opinion on that. > > > > > > > > > > FWIW, I just recalled that I wrote a quick summary: > > > > > > > > > > https://lkml.kernel.org/r/f1783ff0-65bd-4b2b-8952-52b6822a0835@redhat.com > > > > > > > > > > I believe the meetings are recorded as well, but never looked at recordings. > > > > > > > > That's not what I understood Hugh to mean. To me, it seemed that Hugh > > > > was expressing an opinion on using shmem as shmem, not as using it as > > > > tmpfs. > > > > > > > > If I misunderstood Hugh, well, I still disagree. We should not have > > > > separate controls for this. tmpfs is just not that special. > > > > I wasn't at the meeting that's being referred to, but I thought we previously > > agreed that tmpfs *is* special because in some configurations its not backed by > > swap so is locked in ram? > > There are multiple things to that, like: > > * Machines only having limited/no swap configured > * tmpfs can be configured to never go to swap > * memfd/tmpfs files getting used purely for mmap(): there is no real > difference to MAP_ANON|MAP_SHARE besides the processes we share that > memory with. > > Especially when it comes to memory waste concerns and access behavior in > some cases, tmpfs behaved much more like anonymous memory. But there are for > sure other use cases where tmpfs is not that special. Having controls to select the allowable folio order allocations for tmpfs does not address any of these issues. The suggested filesystem approach [1] involves allocating orders in larger chunks, but always the same size you would allocate when using order-0 folios. So, it's a conservative approach. Using mTHP knobs in tmpfs would cause: * Over allocation when using mTHP and/ord THP under the 'always' flag. * Allocate in bigger chunks in a non optimal way, when not all mTHP and THP orders are enabled. * Operate in a similar manner as in [1] when all mTHP and THP orders are enabled and 'within_size' flag is used (assuming we use patch 11 from [1]). [1] Last 3 patches of these series: https://lore.kernel.org/all/20240515055719.32577-1-da.gomez@samsung.com/ My understanding of why mTHP was preferred is to raise awareness in user space and allow tmpfs mounts used at boot time to operate in 'safe' mode (no large folios). Does it make more sense to have a large folios enable flag to control order allocation as in [1], instead of every single order possible? > > My opinion is that we need to let people configure orders (if you feel like > it, configure all), but *select* the order to allocate based on readahead > information -- in contrast to anonymous memory where we start at the highest > order and don't have readahead information available. > > Maybe we need different "order allcoation" logic for read/write vs. fault, > not sure. I would suggest [1] the file size of the write for the write and fallocate paths. But when does make sense to use readahead information? Maybe when swap is involved? > > But I don't maintain that code, so I can only give stupid suggestions and > repeat what I understood from the meeting with Hugh and Kirill :) > > -- > Cheers, > > David / dhildenb >
On 07/07/2024 17:39, Daniel Gomez wrote: > On Fri, Jul 05, 2024 at 10:59:02AM GMT, David Hildenbrand wrote: >> On 05.07.24 10:45, Ryan Roberts wrote: >>> On 05/07/2024 06:47, Baolin Wang wrote: >>>> >>>> >>>> On 2024/7/5 03:49, Matthew Wilcox wrote: >>>>> On Thu, Jul 04, 2024 at 09:19:10PM +0200, David Hildenbrand wrote: >>>>>> On 04.07.24 21:03, David Hildenbrand wrote: >>>>>>>> shmem has two uses: >>>>>>>> >>>>>>>> - MAP_ANONYMOUS | MAP_SHARED (this patch set) >>>>>>>> - tmpfs >>>>>>>> >>>>>>>> For the second use case we don't want controls *at all*, we want the >>>>>>>> same heiristics used for all other filesystems to apply to tmpfs. >>>>>>> >>>>>>> As discussed in the MM meeting, Hugh had a different opinion on that. >>>>>> >>>>>> FWIW, I just recalled that I wrote a quick summary: >>>>>> >>>>>> https://lkml.kernel.org/r/f1783ff0-65bd-4b2b-8952-52b6822a0835@redhat.com >>>>>> >>>>>> I believe the meetings are recorded as well, but never looked at recordings. >>>>> >>>>> That's not what I understood Hugh to mean. To me, it seemed that Hugh >>>>> was expressing an opinion on using shmem as shmem, not as using it as >>>>> tmpfs. >>>>> >>>>> If I misunderstood Hugh, well, I still disagree. We should not have >>>>> separate controls for this. tmpfs is just not that special. >>> >>> I wasn't at the meeting that's being referred to, but I thought we previously >>> agreed that tmpfs *is* special because in some configurations its not backed by >>> swap so is locked in ram? >> >> There are multiple things to that, like: >> >> * Machines only having limited/no swap configured >> * tmpfs can be configured to never go to swap >> * memfd/tmpfs files getting used purely for mmap(): there is no real >> difference to MAP_ANON|MAP_SHARE besides the processes we share that >> memory with. >> >> Especially when it comes to memory waste concerns and access behavior in >> some cases, tmpfs behaved much more like anonymous memory. But there are for >> sure other use cases where tmpfs is not that special. > > Having controls to select the allowable folio order allocations for > tmpfs does not address any of these issues. The suggested filesystem > approach [1] involves allocating orders in larger chunks, but always > the same size you would allocate when using order-0 folios. Well you can't know that you will never allocate more. If you allocate a 2M block, you probably have some good readahead data that tells you you are likely to keep reading sequentially, but you don't know for sure that the application won't stop after just 4K. > So, > it's a conservative approach. Using mTHP knobs in tmpfs would cause: > * Over allocation when using mTHP and/ord THP under the 'always' flag. > * Allocate in bigger chunks in a non optimal way, when > not all mTHP and THP orders are enabled. > * Operate in a similar manner as in [1] when all mTHP and THP orders > are enabled and 'within_size' flag is used (assuming we use patch 11 > from [1]). Large folios may still be considered scarce resources even if the amount of memory allocated is still the same. And if shmem isn't backed by swap then once you have allocated a large folio for shmem, it is stuck in shmem, even if it would be better used somewhere else. And it's possible (likely even, in my opinion) that allocating lots of different folio sizes will exacerbate memory fragmentation, leading to more order-0 fallbacks, which would hurt the overall system performance in the long run, vs restricting to a couple of folio sizes. I'm starting some work to actually measure how limiting the folio sizes allocated for page cache memory can help reduce large folio allocation failure overall. My hypothesis is that the data will show us that in an environment like Android, where memory pressure is high, limiting everything to order-0 and order-4 will significantly improve the allocation success rate of order-4. Let's see. > > [1] Last 3 patches of these series: > https://lore.kernel.org/all/20240515055719.32577-1-da.gomez@samsung.com/ > > My understanding of why mTHP was preferred is to raise awareness in > user space and allow tmpfs mounts used at boot time to operate in > 'safe' mode (no large folios). Does it make more sense to have a large > folios enable flag to control order allocation as in [1], instead of > every single order possible? My intuition is towards every order possible, as per above. Let's see what the data tells us. > >> >> My opinion is that we need to let people configure orders (if you feel like >> it, configure all), but *select* the order to allocate based on readahead >> information -- in contrast to anonymous memory where we start at the highest >> order and don't have readahead information available. >> >> Maybe we need different "order allcoation" logic for read/write vs. fault, >> not sure. > > I would suggest [1] the file size of the write for the write > and fallocate paths. But when does make sense to use readahead > information? Maybe when swap is involved? > >> >> But I don't maintain that code, so I can only give stupid suggestions and >> repeat what I understood from the meeting with Hugh and Kirill :) >> >> -- >> Cheers, >> >> David / dhildenb
On Tue, Jul 09, 2024 at 09:28:48AM GMT, Ryan Roberts wrote: > On 07/07/2024 17:39, Daniel Gomez wrote: > > On Fri, Jul 05, 2024 at 10:59:02AM GMT, David Hildenbrand wrote: > >> On 05.07.24 10:45, Ryan Roberts wrote: > >>> On 05/07/2024 06:47, Baolin Wang wrote: > >>>> > >>>> > >>>> On 2024/7/5 03:49, Matthew Wilcox wrote: > >>>>> On Thu, Jul 04, 2024 at 09:19:10PM +0200, David Hildenbrand wrote: > >>>>>> On 04.07.24 21:03, David Hildenbrand wrote: > >>>>>>>> shmem has two uses: > >>>>>>>> > >>>>>>>> - MAP_ANONYMOUS | MAP_SHARED (this patch set) > >>>>>>>> - tmpfs > >>>>>>>> > >>>>>>>> For the second use case we don't want controls *at all*, we want the > >>>>>>>> same heiristics used for all other filesystems to apply to tmpfs. > >>>>>>> > >>>>>>> As discussed in the MM meeting, Hugh had a different opinion on that. > >>>>>> > >>>>>> FWIW, I just recalled that I wrote a quick summary: > >>>>>> > >>>>>> https://lkml.kernel.org/r/f1783ff0-65bd-4b2b-8952-52b6822a0835@redhat.com > >>>>>> > >>>>>> I believe the meetings are recorded as well, but never looked at recordings. > >>>>> > >>>>> That's not what I understood Hugh to mean. To me, it seemed that Hugh > >>>>> was expressing an opinion on using shmem as shmem, not as using it as > >>>>> tmpfs. > >>>>> > >>>>> If I misunderstood Hugh, well, I still disagree. We should not have > >>>>> separate controls for this. tmpfs is just not that special. > >>> > >>> I wasn't at the meeting that's being referred to, but I thought we previously > >>> agreed that tmpfs *is* special because in some configurations its not backed by > >>> swap so is locked in ram? > >> > >> There are multiple things to that, like: > >> > >> * Machines only having limited/no swap configured > >> * tmpfs can be configured to never go to swap > >> * memfd/tmpfs files getting used purely for mmap(): there is no real > >> difference to MAP_ANON|MAP_SHARE besides the processes we share that > >> memory with. > >> > >> Especially when it comes to memory waste concerns and access behavior in > >> some cases, tmpfs behaved much more like anonymous memory. But there are for > >> sure other use cases where tmpfs is not that special. > > > > Having controls to select the allowable folio order allocations for > > tmpfs does not address any of these issues. The suggested filesystem > > approach [1] involves allocating orders in larger chunks, but always > > the same size you would allocate when using order-0 folios. > > Well you can't know that you will never allocate more. If you allocate a 2M In the fs large folio approach implementation [1], the allocation of a 2M (or any non order-0) occurs when the size of the write/fallocate is 2M (and index is aligned). > block, you probably have some good readahead data that tells you you are likely > to keep reading sequentially, but you don't know for sure that the application > won't stop after just 4K. Is shmem_file_read_iter() getting readahead data to perform the read? or what do you mean exactly? In [1], read is perform in chunks of 4k, so I think this does not apply. > > > So, > > it's a conservative approach. Using mTHP knobs in tmpfs would cause: > > * Over allocation when using mTHP and/ord THP under the 'always' flag. > > * Allocate in bigger chunks in a non optimal way, when > > not all mTHP and THP orders are enabled. > > * Operate in a similar manner as in [1] when all mTHP and THP orders > > are enabled and 'within_size' flag is used (assuming we use patch 11 > > from [1]). > > Large folios may still be considered scarce resources even if the amount of > memory allocated is still the same. And if shmem isn't backed by swap then once > you have allocated a large folio for shmem, it is stuck in shmem, even if it > would be better used somewhere else. Is that true for tmpfs as well? We have shmem_unused_huge_shrink() that will reclaim unused large folios (when ENOSPC and free_cached_objects()). Can't we reuse that when the system is under memory pressure? > > And it's possible (likely even, in my opinion) that allocating lots of different > folio sizes will exacerbate memory fragmentation, leading to more order-0 > fallbacks, which would hurt the overall system performance in the long run, vs > restricting to a couple of folio sizes. Since we are transitioning to large folios in other filesystems, the impact of restricting the order here will only depend on the extent of tmpfs usage relative to the rest of the system. Luis discussed the topic of mm fragmentation and measurment in a session at LSFMM this year [2]. [2] https://lore.kernel.org/all/ZkUOXQvVjXP1T6Nk@bombadil.infradead.org/ > > I'm starting some work to actually measure how limiting the folio sizes > allocated for page cache memory can help reduce large folio allocation failure It would be great to hear more about that effort. > overall. My hypothesis is that the data will show us that in an environment like > Android, where memory pressure is high, limiting everything to order-0 and > order-4 will significantly improve the allocation success rate of order-4. Let's > see. > > > > > [1] Last 3 patches of these series: > > https://lore.kernel.org/all/20240515055719.32577-1-da.gomez@samsung.com/ > > > > My understanding of why mTHP was preferred is to raise awareness in > > user space and allow tmpfs mounts used at boot time to operate in > > 'safe' mode (no large folios). Does it make more sense to have a large > > folios enable flag to control order allocation as in [1], instead of > > every single order possible? > > My intuition is towards every order possible, as per above. Let's see what the > data tells us. > > > > >> > >> My opinion is that we need to let people configure orders (if you feel like > >> it, configure all), but *select* the order to allocate based on readahead > >> information -- in contrast to anonymous memory where we start at the highest > >> order and don't have readahead information available. > >> > >> Maybe we need different "order allcoation" logic for read/write vs. fault, > >> not sure. > > > > I would suggest [1] the file size of the write for the write > > and fallocate paths. But when does make sense to use readahead > > information? Maybe when swap is involved? > > > >> > >> But I don't maintain that code, so I can only give stupid suggestions and > >> repeat what I understood from the meeting with Hugh and Kirill :) > >> > >> -- > >> Cheers, > >> > >> David / dhildenb >
On 16.07.24 15:11, Daniel Gomez wrote: > On Tue, Jul 09, 2024 at 09:28:48AM GMT, Ryan Roberts wrote: >> On 07/07/2024 17:39, Daniel Gomez wrote: >>> On Fri, Jul 05, 2024 at 10:59:02AM GMT, David Hildenbrand wrote: >>>> On 05.07.24 10:45, Ryan Roberts wrote: >>>>> On 05/07/2024 06:47, Baolin Wang wrote: >>>>>> >>>>>> >>>>>> On 2024/7/5 03:49, Matthew Wilcox wrote: >>>>>>> On Thu, Jul 04, 2024 at 09:19:10PM +0200, David Hildenbrand wrote: >>>>>>>> On 04.07.24 21:03, David Hildenbrand wrote: >>>>>>>>>> shmem has two uses: >>>>>>>>>> >>>>>>>>>> - MAP_ANONYMOUS | MAP_SHARED (this patch set) >>>>>>>>>> - tmpfs >>>>>>>>>> >>>>>>>>>> For the second use case we don't want controls *at all*, we want the >>>>>>>>>> same heiristics used for all other filesystems to apply to tmpfs. >>>>>>>>> >>>>>>>>> As discussed in the MM meeting, Hugh had a different opinion on that. >>>>>>>> >>>>>>>> FWIW, I just recalled that I wrote a quick summary: >>>>>>>> >>>>>>>> https://lkml.kernel.org/r/f1783ff0-65bd-4b2b-8952-52b6822a0835@redhat.com >>>>>>>> >>>>>>>> I believe the meetings are recorded as well, but never looked at recordings. >>>>>>> >>>>>>> That's not what I understood Hugh to mean. To me, it seemed that Hugh >>>>>>> was expressing an opinion on using shmem as shmem, not as using it as >>>>>>> tmpfs. >>>>>>> >>>>>>> If I misunderstood Hugh, well, I still disagree. We should not have >>>>>>> separate controls for this. tmpfs is just not that special. >>>>> >>>>> I wasn't at the meeting that's being referred to, but I thought we previously >>>>> agreed that tmpfs *is* special because in some configurations its not backed by >>>>> swap so is locked in ram? >>>> >>>> There are multiple things to that, like: >>>> >>>> * Machines only having limited/no swap configured >>>> * tmpfs can be configured to never go to swap >>>> * memfd/tmpfs files getting used purely for mmap(): there is no real >>>> difference to MAP_ANON|MAP_SHARE besides the processes we share that >>>> memory with. >>>> >>>> Especially when it comes to memory waste concerns and access behavior in >>>> some cases, tmpfs behaved much more like anonymous memory. But there are for >>>> sure other use cases where tmpfs is not that special. >>> >>> Having controls to select the allowable folio order allocations for >>> tmpfs does not address any of these issues. The suggested filesystem >>> approach [1] involves allocating orders in larger chunks, but always >>> the same size you would allocate when using order-0 folios. >> >> Well you can't know that you will never allocate more. If you allocate a 2M > > In the fs large folio approach implementation [1], the allocation of a 2M (or > any non order-0) occurs when the size of the write/fallocate is 2M (and index > is aligned). I don't have time right now follow the discussion in detail here (I thought we had a meeting to discuss that and received guidance from Hugh?), but I'll point out two things: (1) We need a reasonable model for handling/allocating of large folios during page faults. shmem/tmpfs can be used just like anon-shmem if you simply only mmap that thing (hello VMs!). (2) Hugh gave (IMHO) clear feedback during the meeting how he thinks we should approach large folios in shmem. Maybe I got (2) all wrong and people can point out all the issues in my summary from the meeting. Otherwise, if people don't want to accept the result from that meeting, we need further guidance from Hugh.