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

[Suren Baghdasaryan]

On Fri, May 20, 2022 at 8:55 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
>
> On Fri, May 20, 2022 at 12:21 AM Michal Hocko <mhocko@xxxxxxxx> wrote:
> >
> > On Thu 19-05-22 14:33:03, Suren Baghdasaryan wrote:
> > > On Thu, May 19, 2022 at 1:22 PM Liam Howlett <liam.howlett@xxxxxxxxxx> 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.
>
> Ah, thanks! I didn't realize these page tables are not reachable from
> VMAs. The only other call that I'm not sure is ok without mmap write
> lock is xen_hvm_exit_mmap:
>
> arch_exit_mmap
>     paravirt_arch_exit_mmap
>         xen_hvm_exit_mmap
>
> I'll look closer today but if someone can confirm it's safe then my
> current patch should be fine as is.

My conclusion is that it's safe to call arch_exit_mmap without
exclusive mmap lock since the only possible competition is from
OOM-killer/process_mrelease which operate on mm->mmap and none of the
arch_exit_mmap implementations use mm->mmap.

Andrew, sorry for going back and forth. I think the patch is fine as
is and can be integrated. Thanks!


> Thanks,
> Suren.
>
> >
> > > 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