Re: [patch v2] mm, oom: fix concurrent munlock and oom reaper unmap

David Rientjes
2018-04-22T03:22:59Z

On Fri, 20 Apr 2018, Michal Hocko wrote:

> diff --git a/mm/mmap.c b/mm/mmap.c
> index faf85699f1a1..216efa6d9f61 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -3004,10 +3004,21 @@ void exit_mmap(struct mm_struct *mm)
> struct mmu_gather tlb;
> struct vm_area_struct *vma;
> unsigned long nr_accounted = 0;
> + bool locked = false;
>
> /* mm's last user has gone, and its about to be pulled down */
> mmu_notifier_release(mm);
>
> + /*
> + * The mm is not accessible for anybody except for the oom reaper
> + * which cannot race with munlocking so make sure we exclude the
> + * two.
> + */
> + if (unlikely(mm_is_oom_victim(mm))) {
> + down_write(&mm->mmap_sem);
> + locked = true;
> + }
> +
> if (mm->locked_vm) {
> vma = mm->mmap;
> while (vma) {
> @@ -3021,7 +3032,7 @@ void exit_mmap(struct mm_struct *mm)
>
> vma = mm->mmap;
> if (!vma) /* Can happen if dup_mmap() received an OOM */
> - return;
> + goto out_unlock;
>
> lru_add_drain();
> flush_cache_mm(mm);
> @@ -3030,23 +3041,6 @@ void exit_mmap(struct mm_struct *mm)
> /* Use -1 here to ensure all VMAs in the mm are unmapped */
> unmap_vmas(&tlb, vma, 0, -1);
>
> - if (unlikely(mm_is_oom_victim(mm))) {
> - /*
> - * Wait for oom_reap_task() to stop working on this
> - * mm. Because MMF_OOM_SKIP is already set before
> - * calling down_read(), oom_reap_task() will not run
> - * on this "mm" post up_write().
> - *
> - * mm_is_oom_victim() cannot be set from under us
> - * either because victim->mm is already set to NULL
> - * under task_lock before calling mmput and oom_mm is
> - * set not NULL by the OOM killer only if victim->mm
> - * is found not NULL while holding the task_lock.
> - */
> - set_bit(MMF_OOM_SKIP, &mm->flags);
> - down_write(&mm->mmap_sem);
> - up_write(&mm->mmap_sem);
> - }
> free_pgtables(&tlb, vma, FIRST_USER_ADDRESS, USER_PGTABLES_CEILING);
> tlb_finish_mmu(&tlb, 0, -1);
>
> @@ -3060,6 +3054,12 @@ void exit_mmap(struct mm_struct *mm)
> vma = remove_vma(vma);
> }
> vm_unacct_memory(nr_accounted);
> +
> +out_unlock:
> + if (unlikely(locked)) {
> + set_bit(MMF_OOM_SKIP, &mm->flags);
> + up_write(&mm->mmap_sem);
> + }
> }
>
> /* Insert vm structure into process list sorted by address

How have you tested this?

I'm wondering why you do not see oom killing of many processes if the
victim is a very large process that takes a long time to free memory in
exit_mmap() as I do because the oom reaper gives up trying to acquire
mm->mmap_sem and just sets MMF_OOM_SKIP itself.