The patch titled
Subject: mm/filemap: don't call folio_test_locked() without a reference in next_uptodate_folio()
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
mm-filemap-dont-call-folio_test_locked-without-a-reference-in-next_uptodate_folio.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-filemap-dont-call-folio_test_locked-without-a-reference-in-next_uptodate_folio.patch
This patch will later appear in the mm-hotfixes-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: David Hildenbrand <david@xxxxxxxxxx>
Subject: mm/filemap: don't call folio_test_locked() without a reference in next_uptodate_folio()
Date: Fri, 29 Nov 2024 13:53:03 +0100
The folio can get freed + buddy-merged + reallocated in the meantime,
resulting in us calling folio_test_locked() possibly on a tail page.
This makes const_folio_flags VM_BUG_ON_PGFLAGS() when stumbling over the
tail page.
Could this result in other issues? Doesn't look like it. False positives
and false negatives don't really matter, because this folio would get
skipped either way when detecting that they have been reallocated in the
meantime.
Fix it by performing the folio_test_locked() checked after grabbing a
reference. If this ever becomes a real problem, we could add a special
helper that racily checks if the bit is set even on tail pages ... but
let's hope that's not required so we can just handle it cleaner: work on
the folio after we hold a reference.
Do we really need the folio_test_locked() check if we are going to trylock
briefly after? Well, we can at least avoid a xas_reload().
It's a bit unclear which exact change introduced that issue. Likely, ever
since we made PG_locked obey to the PF_NO_TAIL policy it could have been
triggered in some way.
Link: https://lkml.kernel.org/r/20241129125303.4033164-1-david@xxxxxxxxxx
Fixes: 48c935ad88f5 ("page-flags: define PG_locked behavior on compound pages")
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
Reported-by: syzbot+9f9a7f73fb079b2387a6@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://lore.kernel.org/lkml/674184c9.050a0220.1cc393.0001.GAE@xxxxxxxxxx/
Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
Cc: Hillf Danton <hdanton@xxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/filemap.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
--- a/mm/filemap.c~mm-filemap-dont-call-folio_test_locked-without-a-reference-in-next_uptodate_folio
+++ a/mm/filemap.c
@@ -3501,10 +3501,10 @@ static struct folio *next_uptodate_folio
continue;
if (xa_is_value(folio))
continue;
- if (folio_test_locked(folio))
- continue;
if (!folio_try_get(folio))
continue;
+ if (folio_test_locked(folio))
+ goto skip;
/* Has the page moved or been split? */
if (unlikely(folio != xas_reload(xas)))
goto skip;
_
Patches currently in -mm which might be from david@xxxxxxxxxx are
mm-mempolicy-fix-migrate_to_node-assuming-there-is-at-least-one-vma-in-a-mm.patch
mm-filemap-dont-call-folio_test_locked-without-a-reference-in-next_uptodate_folio.patch
docs-tmpfs-update-the-large-folios-policy-for-tmpfs-and-shmem.patch
