Re: [PATCH v2 1/2] mm: drop oom code from exit_mmap

[Michal Hocko <mhocko@suse.com>]

On Thu 19-05-22 14:33:03, Suren Baghdasaryan wrote:
> On Thu, May 19, 2022 at 1:22 PM Liam Howlett <liam.howlett@oracle.com> wrote:
[...]
> > arch_exit_mmap() was called under the write lock before, is it safe to
> > call it under the read lock?
> 
> Ah, good catch. I missed at least one call chain which I believe would
> require arch_exit_mmap() to be called under write lock:
> 
> arch_exit_mmap
>     ldt_arch_exit_mmap
>         free_ldt_pgtables
>             free_pgd_range

Why would be this a problem? This is LDT mapped into page tables but as
far as I know oom_reaper cannot really ever see that range because it is
not really reachable from any VMA.

> I'll need to check whether arch_exit_mmap() has to be called before
> unmap_vmas(). If not, we could move it further down when we hold the
> write lock.
> Andrew, please remove this patchset from your tree for now until I fix this.
> 
> >
> > >
> > >       vma = mm->mmap;
> > >       if (!vma) {
> > >               /* Can happen if dup_mmap() received an OOM */
> > > -             mmap_write_unlock(mm);
> > > +             mmap_read_unlock(mm);
> > >               return;
> > >       }
> > >
> > > @@ -3138,6 +3121,16 @@ void exit_mmap(struct mm_struct *mm)
> > >       /* update_hiwater_rss(mm) here? but nobody should be looking */
> > >       /* Use -1 here to ensure all VMAs in the mm are unmapped */
> > >       unmap_vmas(&tlb, vma, 0, -1);
> > > +     mmap_read_unlock(mm);
> > > +
> > > +     /*
> > > +      * Set MMF_OOM_SKIP to hide this task from the oom killer/reaper
> > > +      * because the memory has been already freed. Do not bother checking
> > > +      * mm_is_oom_victim because setting a bit unconditionally is cheaper.
> > > +      */
> > > +     set_bit(MMF_OOM_SKIP, &mm->flags);
> > > +
> > > +     mmap_write_lock(mm);
> >
> > Is there a race here?  We had a VMA but after the read lock was dropped,
> > could the oom killer cause the VMA to be invalidated?

Nope, the oom killer itself doesn't do much beyond sending SIGKILL and
scheduling the victim for the oom_reaper. dup_mmap is holding exclusive
mmap_lock throughout the whole process.

> > I don't think so
> > but the comment above about dup_mmap() receiving an OOM makes me
> > question it.  The code before kept the write lock from when the VMA was
> > found until the end of the mm edits - and it had the check for !vma
> > within the block itself.  We are also hiding it from the oom killer
> > outside the read lock so it is possible for oom to find it in that
> > window, right?

The oom killer's victim selection doesn't really depend on the
mmap_lock. If there is a race and MMF_OOM_SKIP is not set yet then it
will consider the task and very likely bail out anyway because the
address space has already been unampped so oom_badness() would consider
this task boring.

oom_reaper on the other hand would just try to unmap in parallel but
that is fine regardless of MMF_OOM_SKIP. Seeing the flag would allow to
bail out early rather than just trying to unmap something that is no
longer there. The only problem for the oom_reaper is to see page tables
of the address space disappearing from udner its feet. That is excluded
by the the exlusive lock and as Suren mentions mm->mmap == NULL check
if the exit_mmap wins the race.
-- 
Michal Hocko
SUSE Labs