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    SubjectRe: [PATCH v4 3/3] mm/oom_kill: allow process_mrelease to run under mmap_lock protection
    On Wed 08-12-21 13:22:11, Suren Baghdasaryan wrote:
    > With exit_mmap holding mmap_write_lock during free_pgtables call,
    > process_mrelease does not need to elevate mm->mm_users in order to
    > prevent exit_mmap from destrying pagetables while __oom_reap_task_mm
    > is walking the VMA tree. The change prevents process_mrelease from
    > calling the last mmput, which can lead to waiting for IO completion
    > in exit_aio.
    >
    > Fixes: 337546e83fc7 ("mm/oom_kill.c: prevent a race between process_mrelease and exit_mmap")

    I am not sure I have brought this up already but I do not think Fixes
    tag is a good fit. 337546e83fc7 is a correct way to handle the race. It
    is just slightly less optimal than this fix.

    > Signed-off-by: Suren Baghdasaryan <surenb@google.com>

    Acked-by: Michal Hocko <mhocko@suse.com>

    Thanks!
    > ---
    > mm/oom_kill.c | 27 +++++++++++++++------------
    > 1 file changed, 15 insertions(+), 12 deletions(-)
    >
    > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
    > index 1ddabefcfb5a..67780386f478 100644
    > --- a/mm/oom_kill.c
    > +++ b/mm/oom_kill.c
    > @@ -1169,15 +1169,15 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
    > goto put_task;
    > }
    >
    > - if (mmget_not_zero(p->mm)) {
    > - mm = p->mm;
    > - if (task_will_free_mem(p))
    > - reap = true;
    > - else {
    > - /* Error only if the work has not been done already */
    > - if (!test_bit(MMF_OOM_SKIP, &mm->flags))
    > - ret = -EINVAL;
    > - }
    > + mm = p->mm;
    > + mmgrab(mm);
    > +
    > + if (task_will_free_mem(p))
    > + reap = true;
    > + else {
    > + /* Error only if the work has not been done already */
    > + if (!test_bit(MMF_OOM_SKIP, &mm->flags))
    > + ret = -EINVAL;
    > }
    > task_unlock(p);
    >
    > @@ -1188,13 +1188,16 @@ SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
    > ret = -EINTR;
    > goto drop_mm;
    > }
    > - if (!__oom_reap_task_mm(mm))
    > + /*
    > + * Check MMF_OOM_SKIP again under mmap_read_lock protection to ensure
    > + * possible change in exit_mmap is seen
    > + */
    > + if (!test_bit(MMF_OOM_SKIP, &mm->flags) && !__oom_reap_task_mm(mm))
    > ret = -EAGAIN;
    > mmap_read_unlock(mm);
    >
    > drop_mm:
    > - if (mm)
    > - mmput(mm);
    > + mmdrop(mm);
    > put_task:
    > put_task_struct(task);
    > return ret;
    > --
    > 2.34.1.400.ga245620fadb-goog

    --
    Michal Hocko
    SUSE Labs

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