Re: mm/huge_memory: do not clobber swp_entry_t during THP split

Mel Gorman
2022-10-25T10:03:43Z

Cc'ing Andrew for awareness. Andrew, this bug report is almost identical to
the one Hugh already reported and fixed in "[PATCH] mm: prep_compound_tail()
clear page->private". Nothing wrong with the patch AFAIK and only the last
paragraph is relevant to you.

On Tue, Oct 25, 2022 at 09:50:14AM +0100, Tvrtko Ursulin wrote:
>
> On 24/10/2022 15:23, Mel Gorman wrote:
> > On Mon, Oct 24, 2022 at 02:04:50PM +0100, Tvrtko Ursulin wrote:
> > >
> > > Hi Mel, mm experts,
> > >
> > > With 6.1-rc2 we started hitting the WARN_ON added in 71e2d666ef85 ("mm/huge_memory: do not clobber swp_entry_t during THP split") in i915 automated CI:
> > >
> >
> > Thanks for the report. As shmem pages pages are allocated via vma_alloc_folio
> > and are compound pages, can you try the following patch please? If it
> > still triggers, please post the new oops as it'll include the tail page
> > information.
> >
> > --8<--
> > From: Hugh Dickins <hughd@google.com>
> > Subject: [PATCH] mm: prep_compound_tail() clear page->private
> >
> > Although page allocation always clears page->private in the first page
> > or head page of an allocation, it has never made a point of clearing
> > page->private in the tails (though 0 is often what is already there).
> >
> > But now commit 71e2d666ef85 ("mm/huge_memory: do not clobber swp_entry_t
> > during THP split") issues a warning when page_tail->private is found to
> > be non-0 (unless it's swapcache).
> >
> > Change that warning to dump page_tail (which also dumps head), instead
> > of just the head: so far we have seen dead000000000122, dead000000000003,
> > dead000000000001 or 0000000000000002 in the raw output for tail private.
> >
> > We could just delete the warning, but today's consensus appears to want
> > page->private to be 0, unless there's a good reason for it to be set:
> > so now clear it in prep_compound_tail() (more general than just for THP;
> > but not for high order allocation, which makes no pass down the tails).
> >
> > Fixes: 71e2d666ef85 ("mm/huge_memory: do not clobber swp_entry_t during THP split")
> > Signed-off-by: Hugh Dickins <hughd@google.com>
> > Cc: Mel Gorman <mgorman@techsingularity.net>
> > Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
> > Cc: <stable@vger.kernel.org>
> > ---
> > mm/huge_memory.c | 2 +-
> > mm/page_alloc.c | 1 +
> > 2 files changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 03fc7e5edf07..561a42567477 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -2462,7 +2462,7 @@ static void __split_huge_page_tail(struct page *head, int tail,
> > * Fix up and warn once if private is unexpectedly set.
> > */
> > if (!folio_test_swapcache(page_folio(head))) {
> > - VM_WARN_ON_ONCE_PAGE(page_tail->private != 0, head);
> > + VM_WARN_ON_ONCE_PAGE(page_tail->private != 0, page_tail);
> > page_tail->private = 0;
> > }
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index b5a6c815ae28..218b28ee49ed 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -807,6 +807,7 @@ static void prep_compound_tail(struct page *head, int tail_idx)
> > p->mapping = TAIL_MAPPING;
> > set_compound_head(p, head);
> > + set_page_private(p, 0);
> > }
> > void prep_compound_page(struct page *page, unsigned int order)
>
> The patch seems to fix our CI runs.

Thanks for letting me know.

> Is it considered final version?

AFAIK, yes.

> If so I
> can temporarily put it in until it arrives via the next rc - assuming that
> would be the flow from upstream pov?
>

I expect it to. It's currently in the akpm/mm.git branch
mm/mm-hotfixes-unstable where I expect it to flow to mm/mm-hotfixes-stable
in due course before sending to Linus. I can't make promises about the
timing as that's determined by Andrew.

--
Mel Gorman
SUSE Labs