Re: [PATCH] mm: rmap: don't try to add an unevictable page to lru list

[Hugh Dickins <hughd@google.com>]

On Tue, 8 Apr 2014, Bob Liu wrote:
> On Sat, Apr 5, 2014 at 5:04 PM, Hugh Dickins <hughd@google.com> wrote:
> > On Mon, 31 Mar 2014, Bob Liu wrote:
> >
> >> VM_BUG_ON_PAGE(PageActive(page) && PageUnevictable(page), page) in
> >> lru_cache_add() was triggered during migrate_misplaced_transhuge_page.
> >>...
> >> From vmscan.c:
> >>  * Reasons page might not be evictable:
> >>  * (1) page's mapping marked unevictable
> >>  * (2) page is part of an mlocked VMA
> >>
> >> But page_add_new_anon_rmap() only checks reason (2), we may hit this
> >> VM_BUG_ON_PAGE() if PageUnevictable(old_page) was originally set by reason (1).
> >
> > But (1) always reports evictable on an anon page, doesn't it?
> >
> >>
> >> Reported-by: Sasha Levin <sasha.levin@oracle.com>
> >> Signed-off-by: Bob Liu <bob.liu@oracle.com>
> >
> > I can't quite assert NAK, but I suspect this is not the proper fix.
...
> >
> > (Yet now I'm wavering again: if down_write mmap_sem is needed to
> > munlock() the vma, and migrate_misplaced_transhuge_page() is only
> > migrating a singly-mapped THP under down_read mmap_sem, how could
> > VM_LOCKED have changed during the migration?  I've lost sight of
> 
> I think you are right, I'll do more investigation about why this BUG
> was triggered.

Andrew, if Bob agrees, please drop

mm-rmap-dont-try-to-add-an-unevictable-page-to-lru-list.patch

from mmotm now.  We have not heard any such report yet on 3.15-rc,
and neither Bob nor I have yet come up with a convincing explanation
for how it came about.  It's tempting to suppose it was a side-effect
of something temporarily wrong on a 3.14-next, and now okay; but we'll
learn more quickly whether that's so if mmotm stops working around it.

Thanks,
Hugh

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