Re: [PATCH v7] mm: shrink skip folio mapped by an exiting process

[Barry Song]

On Wed, Jul 10, 2024 at 1:46 PM zhiguojiang <justinjiang@xxxxxxxx> wrote:
>
>
>
> 在 2024/7/9 21:02, Barry Song 写道:
> > On Tue, Jul 9, 2024 at 8:31 PM Zhiguo Jiang <justinjiang@xxxxxxxx> wrote:
> >> The releasing process of the non-shared anonymous folio mapped solely by
> >> an exiting process may go through two flows: 1) the anonymous folio is
> >> firstly is swaped-out into swapspace and transformed into a swp_entry
> >> in shrink_folio_list; 2) then the swp_entry is released in the process
> >> exiting flow. This will result in the high cpu load of releasing a
> >> non-shared anonymous folio mapped solely by an exiting process.
> >>
> >> When the low system memory and the exiting process exist at the same
> >> time, it will be likely to happen, because the non-shared anonymous
> >> folio mapped solely by an exiting process may be reclaimed by
> >> shrink_folio_list.
> >>
> >> This patch is that shrink skips the non-shared anonymous folio solely
> >> mapped by an exting process and this folio is only released directly in
> >> the process exiting flow, which will save swap-out time and alleviate
> >> the load of the process exiting.
> >>
> >> Signed-off-by: Zhiguo Jiang <justinjiang@xxxxxxxx>
> > You should have collected tags such as reviewed-by, acked-by you got in v6
> > while sending v7.
> >
> > Again,
> > Acked-by: Barry Song <baohua@xxxxxxxxxx>
> Yes, it is alreadly included in patch v7.

obviously no. please take a look how people collect tags while sending a
new version:

https://lore.kernel.org/linux-mm/20240704012905.42971-2-ioworker0@xxxxxxxxx/


>
> Thanks
> Zhiguo
> >
> >> ---
> >>
> >> Change log:
> >> v6->v7:
> >> 1.Modify tab indentation to space indentation of the continuation
> >> lines of the condition.
> >> v5->v6:
> >> 1.Move folio_likely_mapped_shared() under the PTL.
> >> 2.Add check_stable_address_space() to replace MMF_OOM_SKIP.
> >> 3.Remove folio_test_anon(folio).
> >> v4->v5:
> >> 1.Further modify to skip non-shared anonymous folio only.
> >> 2.Update comments for pra->referenced = -1.
> >> v3->v4:
> >> 1.Modify to skip only the non-shared anonymous folio mapped solely
> >> by an exiting process.
> >> v2->v3:
> >> Nothing.
> >> v1->v2:
> >> 1.The VM_EXITING added in v1 patch is removed, because it will fail
> >> to compile in 32-bit system.
> >>
> >>
> >> Comments from participants and my responses:
> >> [v6->v7]:
> >> 1.Matthew Wilcox <willy@xxxxxxxxxxxxx>
> >> You told me you'd fix the indentation.  You cannot indent both the
> >> continuation lines of the condition and the body of the if by one tab
> >> each!
> >> -->
> >> Modify tab indentation to space indentation of the continuation
> >> lines of the condition.
> >>
> >> [v5->v6]:
> >> 1.David Hildenbrand <david@xxxxxxxxxx>
> >> I'm currently working on moving all folio_likely_mapped_shared() under
> >> the PTL, where we are then sure that the folio is actually mapped by
> >> this process (e.g., no concurrent unmapping poisslbe). Can we do the
> >> same here directly?
> >> -->
> >> You are right. we might use page_vma_mapped_walk_done() to bail out.
> >> (Barry Song)
> >>
> >> 2.Barry Song <baohua@xxxxxxxxxx>
> >> By the way, I am not convinced that using test_bit(MMF_OOM_SKIP,
> >> &vma->vm_mm->flags) is correct (I think it is wrong). And exit_mmap()
> >> automatically has MMF_OOM_SKIP. What is the purpose of this check?
> >> Is there a better way to determine if a process is an OOM target?
> >> What about check_stable_address_space() ?
> >> -->
> >> Sorry, I overlook the situation with if (is_global_init(p)),
> >> MMF_OOM_SKIP is indeed not suitable. It seems feasible for
> >> check_stable_address_space() replacing MMF_OOM_SKIP.
> >> check_stable_address_space() can indicate oom kill, and
> >> !atomic_read(&vma->vm_mm->mm_users) can indicate the normal
> >> process exiting.
> >>
> >> I also think we actually can remove "folio_test_anon(folio)".
> >> -->
> >> Yes, update in patch v6.
> >>
> >> [v4->v5]:
> >> 1.Barry Song <baohua@xxxxxxxxxx>
> >> I don't think this is correct. folio_likely_mapped_shared() is almost
> >> "correct" but not always.
> >> Please explain why you set  pra->referenced =  -1. Please address all
> >> comments before you send a new version.
> >> -->
> >> Update in patch v5.
> >>
> >> 2.Matthew Wilcox <willy@xxxxxxxxxxxxx>
> >> How is the file folio similar?  File folios are never written to swap,
> >> and they'll be written back from the page cache whenever the filesystem
> >> decides it's a good time to do so.
> >> -->
> >> What do you mean is that the file folio will not have any relevant
> >> identifier left in memory after it is reclamed in the shrink flow,
> >> and it will not be released again during an exiting process? If that's
> >> the case, I think we only need the anon folio is skipped here.
> >>
> >> [v3->v4]:
> >> 1.Barry Song <baohua@xxxxxxxxxx>
> >> This is clearly version 3, as you previously sent version 2, correct?
> >> -->
> >> Yes.
> >>
> >> Could you please describe the specific impact on users, including user
> >> experience and power consumption? How serious is this problem?
> >> -->
> >> At present, I do not have a suitable method to accurately measure the
> >> optimization benefit datas of this modifications, but I believe it
> >> theoretically has some benefits.
> >> Launching large memory app (for example, starting the camera) in multiple
> >> backend scenes may result in the high cpu load of the exiting processes.
> >>
> >> Applications?
> >> -->
> >> Yes, when system is low memory, it more likely to occur.
> >>
> >> I'm not completely convinced this patch is correct, but it appears to be
> >> heading in the right direction. Therefore, I expect to see new versions
> >> rather than it being dead.
> >> You changed the file mode to 755, which is incorrect.
> >> -->
> >> Solved.
> >>
> >> Why use -1? Is this meant to simulate lock contention to keep the folio
> >> without activating it? Please do have some comments to explain why.
> >> I'm not convinced this change is appropriate for shared folios. It seems
> >> more suitable for exclusive folios used solely by the exiting process.
> >> -->
> >> The skiped folios are FOLIOREF_KEEP and added into inactive lru, beacase
> >> the folios will be freed soon in the exiting process flow.
> >> Yes, the shared folios can not be simply skipped. I have made relevant
> >> modifications in patch v4 and please help to further review.
> >> https://lore.kernel.org/linux-mm/20240708031517.856-1-justinjiang@xxxxxxxx/
> >>
> >> 2.David Hildenbrand <david@xxxxxxxxxx>
> >> but what if it is shared among multiple processes and only one of them
> >> is exiting?
> >> -->
> >> Modify to skip only the non-shared anonymous folio mapped solely
> >> by an exiting process in next version v4.
> >>
> >> [v2->v3:]
> >> Nothing.
> >>
> >> [v1->v2]:
> >> 1.Matthew Wilcox <willy@xxxxxxxxxxxxx>
> >> What testing have you done of this patch?  How often does it happen?
> >> Are there particular workloads that benefit from this?  (I'm not sure
> >> what "mutil backed-applications" are?)
> >> And I do mean specifically of this patch, because to my eyes it
> >> shouldn't even compile. Except on 32-bit where it'll say "warning:
> >> integer constant out of range".
> >> -->
> >> Yes, I have tested. When the low system memory and the exiting process
> >> exist at the same time, it will happen. This modification can alleviate
> >> the load of the exiting process.
> >> "mutil backed-applications" means that there are a large number of
> >> the backend applications in the system.
> >> The VM_EXITING added in v1 patch is removed, because it will fail
> >> to compile in 32-bit system.
> >>
> >>   mm/rmap.c   | 14 ++++++++++++++
> >>   mm/vmscan.c |  7 ++++++-
> >>   2 files changed, 20 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/mm/rmap.c b/mm/rmap.c
> >> index 88156deb46a6..bb9954773cce 100644
> >> --- a/mm/rmap.c
> >> +++ b/mm/rmap.c
> >> @@ -877,6 +877,20 @@ static bool folio_referenced_one(struct folio *folio,
> >>                          continue;
> >>                  }
> >>
> >> +               /*
> >> +                * Skip the non-shared swapbacked folio mapped solely by
> >> +                * the exiting or OOM-reaped process. This avoids redundant
> >> +                * swap-out followed by an immediate unmap.
> >> +                */
> >> +               if ((!atomic_read(&vma->vm_mm->mm_users) ||
> >> +                   check_stable_address_space(vma->vm_mm)) &&
> >> +                   folio_test_swapbacked(folio) &&
> >> +                   !folio_likely_mapped_shared(folio)) {
> >> +                       pra->referenced = -1;
> >> +                       page_vma_mapped_walk_done(&pvmw);
> >> +                       return false;
> >> +               }
> >> +
> >>                  if (pvmw.pte) {
> >>                          if (lru_gen_enabled() &&
> >>                              pte_young(ptep_get(pvmw.pte))) {
> >> diff --git a/mm/vmscan.c b/mm/vmscan.c
> >> index 80f9a486cf27..1d5f78a3dbeb 100644
> >> --- a/mm/vmscan.c
> >> +++ b/mm/vmscan.c
> >> @@ -863,7 +863,12 @@ static enum folio_references folio_check_references(struct folio *folio,
> >>          if (vm_flags & VM_LOCKED)
> >>                  return FOLIOREF_ACTIVATE;
> >>
> >> -       /* rmap lock contention: rotate */
> >> +       /*
> >> +        * There are two cases to consider.
> >> +        * 1) Rmap lock contention: rotate.
> >> +        * 2) Skip the non-shared swapbacked folio mapped solely by
> >> +        *    the exiting or OOM-reaped process.
> >> +        */
> >>          if (referenced_ptes == -1)
> >>                  return FOLIOREF_KEEP;
> >>
> >> --
> >> 2.39.0
> >>
>