Re: [PATCH 4/3] mm: drop MMF_OOM_SKIP from exit_mmap

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Dec 9, 2021 at 1:12 AM Michal Hocko <mhocko@suse.com> wrote:
>
> Do we want this on top?

As we discussed in this thread
https://lore.kernel.org/all/YY4snVzZZZYhbigV@dhcp22.suse.cz,
__oom_reap_task_mm in exit_mmap allows oom-reaper/process_mrelease to
unmap pages in parallel with exit_mmap without blocking each other.
Removal of __oom_reap_task_mm from exit_mmap prevents this parallelism
and has a negative impact on performance. So the conclusion of that
thread I thought was to keep that part. My understanding is that we
also wanted to remove MMF_OOM_SKIP as a follow-up patch but
__oom_reap_task_mm would stay.


> ----
> From 58b04ae6dc97b0105ea2651daca55cf2386f69b4 Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@suse.com>
> Date: Thu, 9 Dec 2021 10:07:51 +0100
> Subject: [PATCH] mm: drop MMF_OOM_SKIP from exit_mmap
>
> MMF_OOM_SKIP used to play a synchronization role between exit_mmap and
> oom repear in the past. Since the exclusive mmap_sem is held in
> exit_mmap to cover all destructive operations the flag synchronization
> is not needed anymore and we can safely drop it. Just make sure that
> mm->mmap is set to NULL so that nobody will access the freed vma list.
>
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
>  mm/mmap.c | 23 +----------------------
>  1 file changed, 1 insertion(+), 22 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index f4e09d390a07..0d6af9d89aa8 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -3129,28 +3129,6 @@ void exit_mmap(struct mm_struct *mm)
>         /* mm's last user has gone, and its about to be pulled down */
>         mmu_notifier_release(mm);
>
> -       if (unlikely(mm_is_oom_victim(mm))) {
> -               /*
> -                * Manually reap the mm to free as much memory as possible.
> -                * Then, as the oom reaper does, set MMF_OOM_SKIP to disregard
> -                * this mm from further consideration.  Taking mm->mmap_lock for
> -                * write after setting MMF_OOM_SKIP will guarantee that the oom
> -                * reaper will not run on this mm again after mmap_lock is
> -                * dropped.
> -                *
> -                * Nothing can be holding mm->mmap_lock here and the above call
> -                * to mmu_notifier_release(mm) ensures mmu notifier callbacks in
> -                * __oom_reap_task_mm() will not block.
> -                *
> -                * This needs to be done before calling unlock_range(),
> -                * which clears VM_LOCKED, otherwise the oom reaper cannot
> -                * reliably test it.
> -                */
> -               (void)__oom_reap_task_mm(mm);
> -
> -               set_bit(MMF_OOM_SKIP, &mm->flags);
> -       }
> -
>         mmap_write_lock(mm);
>         if (mm->locked_vm)
>                 unlock_range(mm->mmap, ULONG_MAX);
> @@ -3180,6 +3158,7 @@ void exit_mmap(struct mm_struct *mm)
>                 vma = remove_vma(vma);
>                 cond_resched();
>         }
> +       mm->mmap = NULL;
>         mmap_write_unlock(mm);
>         vm_unacct_memory(nr_accounted);
>  }
> --
> 2.30.2
>
> --
> Michal Hocko
> SUSE Labs