Re: [PATCH v9 03/10] mm/rmap: Split try_to_munlock from try_to_unmap

[Shakeel Butt]

On Fri, Jun 4, 2021 at 8:39 PM Liam Howlett <liam.howlett@xxxxxxxxxx> wrote:
>
> > Particularly the following paragraph:
> > ...
> >     Vlastimil Babka points out another race which this patch protects against.
> >      try_to_unmap_one() might reach its mlock_vma_page() TestSetPageMlocked a
> >     moment after munlock_vma_pages_all() did its Phase 1 TestClearPageMlocked:
> >     leaving PageMlocked and unevictable when it should be evictable.  mmap_sem
> >     is ineffective because exit_mmap() does not hold it; page lock ineffective
> >     because __munlock_pagevec() only takes it afterwards, in Phase 2; pte lock
> >     is effective because __munlock_pagevec_fill() takes it to get the page,
> >     after VM_LOCKED was cleared from vm_flags, so visible to try_to_unmap_one.
> > ...
>
> So this is saying the race with exit_mmap() isn't benign after all?
>

Yes, not benign at all.