Re: [PATCH 2/4] mm: khugepaged: check if file page is on LRU after locking page

[Yang Shi <shy828301@gmail.com>]

On Wed, Sep 15, 2021 at 10:48 AM Yang Shi <shy828301@gmail.com> wrote:
>
> On Wed, Sep 15, 2021 at 4:49 AM Kirill A. Shutemov <kirill@shutemov.name> wrote:
> >
> > On Tue, Sep 14, 2021 at 11:37:16AM -0700, Yang Shi wrote:
> > > The khugepaged does check if the page is on LRU or not but it doesn't
> > > hold page lock.  And it doesn't check this again after holding page
> > > lock.  So it may race with some others, e.g. reclaimer, migration, etc.
> > > All of them isolates page from LRU then lock the page then do something.
> > >
> > > But it could pass the refcount check done by khugepaged to proceed
> > > collapse.  Typically such race is not fatal.  But if the page has been
> > > isolated from LRU before khugepaged it likely means the page may be not
> > > suitable for collapse for now.
> > >
> > > The other more fatal case is the following patch will keep the poisoned
> > > page in page cache for shmem, so khugepaged may collapse a poisoned page
> > > since the refcount check could pass.  3 refcounts come from:
> > >   - hwpoison
> > >   - page cache
> > >   - khugepaged
> > >
> > > Since it is not on LRU so no refcount is incremented from LRU isolation.
> > >
> > > This is definitely not expected.  Checking if it is on LRU or not after
> > > holding page lock could help serialize against hwpoison handler.
> > >
> > > But there is still a small race window between setting hwpoison flag and
> > > bump refcount in hwpoison handler.  It could be closed by checking
> > > hwpoison flag in khugepaged, however this race seems unlikely to happen
> > > in real life workload.  So just check LRU flag for now to avoid
> > > over-engineering.
> > >
> > > Signed-off-by: Yang Shi <shy828301@gmail.com>
> > > ---
> > >  mm/khugepaged.c | 6 ++++++
> > >  1 file changed, 6 insertions(+)
> > >
> > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > > index 045cc579f724..bdc161dc27dc 100644
> > > --- a/mm/khugepaged.c
> > > +++ b/mm/khugepaged.c
> > > @@ -1808,6 +1808,12 @@ static void collapse_file(struct mm_struct *mm,
> > >                       goto out_unlock;
> > >               }
> > >
> > > +             /* The hwpoisoned page is off LRU but in page cache */
> > > +             if (!PageLRU(page)) {
> > > +                     result = SCAN_PAGE_LRU;
> > > +                     goto out_unlock;
> > > +             }
> > > +
> > >               if (isolate_lru_page(page)) {
> >
> > isolate_lru_page() should catch the case, no? TestClearPageLRU would fail
> > and we get here.
>
> Hmm... you are definitely right. How could I miss this point.
>
> It might be because of I messed up the page state by some tests which
> may do hole punch then reread the same index. That could drop the
> poisoned page then collapse succeed. But I'm not sure. Anyway I didn't
> figure out how the poisoned page could be collapsed. It seems
> impossible. I will drop this patch.

I think I figured out the problem. This problem happened after the
page cache split patch and if the hwpoisoned page is not head page. It
is because THP split will unfreeze the refcount of tail pages to 2
(restore refcount from page cache) then dec refcount to 1. The
refcount pin from hwpoison is gone and it is still on LRU. Then
khugepged locked the page before hwpoison, the refcount is expected to
khugepaged.

The worse thing is it seems this problem is applicable to anonymous
page too. Once the anonymous THP is split by hwpoison the pin from
hwpoison is gone too the refcount is 1 (comes from PTE map). Then
khugepaged could collapse it to huge page again. It may incur data
corruption.

And the poisoned page may be freed back to buddy since the lost refcount pin.

If the poisoned page is head page, the code is fine since hwpoison
doesn't put the refcount for head page after split.

The fix is simple, just keep the refcount pin for hwpoisoned subpage.

>
> >
> > >                       result = SCAN_DEL_PAGE_LRU;
> > >                       goto out_unlock;
> > > --
> > > 2.26.2
> > >
> > >
> >
> > --
> >  Kirill A. Shutemov