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

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