Re: [PATCH v5] mm: shrink skip folio mapped by an exiting process

[zhiguojiang]

在 2024/7/8 19:02, Barry Song 写道:
> On Mon, Jul 8, 2024 at 9:04 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 increase the cpu load of releasing a non-shared
> > anonymous folio mapped solely by an exiting process, because the folio
> > go through swap-out and the releasing the swapspace and swp_entry.
> >
> > When system is low memory, it is more likely to occur, because more
> > backend applidatuions will be killed.
> >
> > The modification is that shrink skips the non-shared anonymous folio
> > solely mapped by an exting process and the 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>
> > ---
> >
> > Change log:
> > v4->v5:
> > 1.Modify to skip non-shared anonymous folio only.
> > 2.Update comments for pra->referenced = -1.
> > v3->v4:
> > 1.Modify that the unshared folios mapped only in exiting task are skip.
> > 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.
> >
> >   mm/rmap.c   | 13 +++++++++++++
> >   mm/vmscan.c |  7 ++++++-
> >   2 files changed, 19 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index 26806b49a86f..5b5281d71dbb
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -843,6 +843,19 @@ static bool folio_referenced_one(struct folio *folio,
> >          int referenced = 0;
> >          unsigned long start = address, ptes = 0;
> >
> > +       /*
> > +        * Skip the non-shared anonymous folio mapped solely by
> > +        * the single exiting process, and release it directly
> > +        * in the process exiting.
> > +        */
> > +       if ((!atomic_read(&vma->vm_mm->mm_users) ||
> > +               test_bit(MMF_OOM_SKIP, &vma->vm_mm->flags)) &&
> > +               folio_test_anon(folio) && folio_test_swapbacked(folio) &&
> > +               !folio_likely_mapped_shared(folio)) {
> > +               pra->referenced = -1;
> > +               return false;
> > +       }
> > +
> >          while (page_vma_mapped_walk(&pvmw)) {
> >                  address = pvmw.address;
Sure, I agree with your modification suggestions. This way, using PTL 
indeed sure
that the folio is mapped by this process.
Thanks
> As David suggested, what about the below?
>
> @@ -883,6 +870,21 @@ static bool folio_referenced_one(struct folio *folio,
>                          continue;
>                  }
>
> +               /*
> +                * Skip the non-shared anonymous folio mapped solely by
> +                * the single exiting process, and release it directly
> +                * in the process exiting.
> +                */
> +               if ((!atomic_read(&vma->vm_mm->mm_users) ||
> +                                       test_bit(MMF_OOM_SKIP,
> &vma->vm_mm->flags)) &&
> +                               folio_test_anon(folio) &&
> 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))) {
>
>
> 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).   For example, global_init can directly have it:
>                  if (is_global_init(p)) {
>                          can_oom_reap = false;
>                          set_bit(MMF_OOM_SKIP, &mm->flags);
>                          pr_info("oom killer %d (%s) has mm pinned by %d (%s)\n",
>                                          task_pid_nr(victim), victim->comm,
>                                          task_pid_nr(p), p->comm);
>                          continue;
>                  }
>
> 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() ?
1.Sorry, I overlook the situation with if (is_global_init(p)), 
MMF_OOM_SKIP is indeed not suitable.

2.check_stable_address_space() can indicate oom_reaper, but it seems 
unable to identify the situation where the process exits normally. What 
about task_is_dying()? static inline bool task_is_dying(void) { return 
tsk_is_oom_victim(current) || fatal_signal_pending(current) || 
(current->flags & PF_EXITING); } Thanks
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index 0761f91b407f..bae7a8bf6b3d
> > --- 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 anonymous folio mapped solely by
> > +        *    the single exiting process.
> > +        */
> >          if (referenced_ptes == -1)
> >                  return FOLIOREF_KEEP;
> >
> > --
> > 2.39.0
> Thanks
> Barry