Re: [PATCH v2 1/2] mm: protect free_pgtables with mmap_lock write lock in exit_mmap

[Michal Hocko <mhocko@suse.com>]

On Mon 06-12-21 10:35:03, Suren Baghdasaryan wrote:
> On Mon, Nov 29, 2021 at 3:23 AM Michal Hocko <mhocko@suse.com> wrote:
> >
> > On Wed 24-11-21 15:59:05, Suren Baghdasaryan wrote:
> > > oom-reaper and process_mrelease system call should protect against
> > > races with exit_mmap which can destroy page tables while they
> > > walk the VMA tree. oom-reaper protects from that race by setting
> > > MMF_OOM_VICTIM and by relying on exit_mmap to set MMF_OOM_SKIP
> > > before taking and releasing mmap_write_lock. process_mrelease has
> > > to elevate mm->mm_users to prevent such race. Both oom-reaper and
> > > process_mrelease hold mmap_read_lock when walking the VMA tree.
> > > The locking rules and mechanisms could be simpler if exit_mmap takes
> > > mmap_write_lock while executing destructive operations such as
> > > free_pgtables.
> > > Change exit_mmap to hold the mmap_write_lock when calling
> > > free_pgtables. Operations like unmap_vmas() and unlock_range() are not
> > > destructive and could run under mmap_read_lock but for simplicity we
> > > take one mmap_write_lock during almost the entire operation. Note
> > > also that because oom-reaper checks VM_LOCKED flag, unlock_range()
> > > should not be allowed to race with it.
> > > In most cases this lock should be uncontended. Previously, Kirill
> > > reported ~4% regression caused by a similar change [1]. We reran the
> > > same test and although the individual results are quite noisy, the
> > > percentiles show lower regression with 1.6% being the worst case [2].
> > > The change allows oom-reaper and process_mrelease to execute safely
> > > under mmap_read_lock without worries that exit_mmap might destroy page
> > > tables from under them.
> > >
> > > [1] https://lore.kernel.org/all/20170725141723.ivukwhddk2voyhuc@node.shutemov.name/
> > > [2] https://lore.kernel.org/all/CAJuCfpGC9-c9P40x7oy=jy5SphMcd0o0G_6U1-+JAziGKG6dGA@mail.gmail.com/
> > >
> > > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > > ---
> > > changes in v2
> > > - Moved mmap_write_unlock to cover remove_vma loop as well, per Matthew Wilcox
> > >
> > >  mm/mmap.c | 16 ++++++++--------
> > >  1 file changed, 8 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/mm/mmap.c b/mm/mmap.c
> > > index bfb0ea164a90..f4e09d390a07 100644
> > > --- a/mm/mmap.c
> > > +++ b/mm/mmap.c
> > > @@ -3142,25 +3142,27 @@ void exit_mmap(struct mm_struct *mm)
> > >                * to mmu_notifier_release(mm) ensures mmu notifier callbacks in
> > >                * __oom_reap_task_mm() will not block.
> > >                *
> > > -              * This needs to be done before calling munlock_vma_pages_all(),
> > > +              * 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);
> >
> > Why do you keep this in place?
> 
> Sorry for the delay, I was out last week.
> I missed your comment about removing MMF_OOM_SKIP at
> https://lore.kernel.org/all/YYrO%2FPwdsyaxJaNZ@dhcp22.suse.cz
> I'll look into removing it in a separate patch, which I think would be cleaner.

The point of this code was to sync up the oom_repaer and exit_mmap. Now
that your patch uses proper locking for that to happen then MMF_OOM_SKIP
is not really necessary. IIRC all you need to guarantee is that the vma
tree is empty when exit_mmap does all its work - i.e set mm->mmap to
NULL. You can do that after remove_vma loop but it would be equally safe
at any time after vma = mm->mmap as the loop relies on the vma chain.
Doing that after would be slightly nicer if you ask me.
 
> > Other than that looks OK to me. Maybe we want to add an explicit note
> > that vm_ops::close cannot take mmap_sem in any form. The changelog
> > should also mention that you have considered remove_vma and its previous
> > no MM locking assumption. You can argue that fput is async and close
> > callback shouldn't really need mmap_sem.
> 
> Should I post another version of this patch with the patch description
> clarifying these points and additional comments as you suggested?

Yes please.

-- 
Michal Hocko
SUSE Labs