The patch titled Subject: mm/gup: check ref_count instead of lru before migration has been added to the -mm mm-new branch. Its filename is mm-gup-check-ref_count-instead-of-lru-before-migration.patch
This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...
This patch will later appear in the mm-new branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
Note, mm-new is a provisional staging ground for work-in-progress patches, and acceptance into mm-new is a notification for others take notice and to finish up reviews. Please do not hesitate to respond to review feedback and post updated versions to replace or incrementally fixup patches in mm-new.
Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's
*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***
The -mm tree is included into linux-next via the mm-everything branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm and is updated there every 2-3 working days
------------------------------------------------------ From: Hugh Dickins hughd@google.com Subject: mm/gup: check ref_count instead of lru before migration Date: Sun, 31 Aug 2025 02:05:56 -0700 (PDT)
Will Deacon reports:-
When taking a longterm GUP pin via pin_user_pages(), __gup_longterm_locked() tries to migrate target folios that should not be longterm pinned, for example because they reside in a CMA region or movable zone. This is done by first pinning all of the target folios anyway, collecting all of the longterm-unpinnable target folios into a list, dropping the pins that were just taken and finally handing the list off to migrate_pages() for the actual migration.
It is critically important that no unexpected references are held on the folios being migrated, otherwise the migration will fail and pin_user_pages() will return -ENOMEM to its caller. Unfortunately, it is relatively easy to observe migration failures when running pKVM (which uses pin_user_pages() on crosvm's virtual address space to resolve stage-2 page faults from the guest) on a 6.15-based Pixel 6 device and this results in the VM terminating prematurely.
In the failure case, 'crosvm' has called mlock(MLOCK_ONFAULT) on its mapping of guest memory prior to the pinning. Subsequently, when pin_user_pages() walks the page-table, the relevant 'pte' is not present and so the faulting logic allocates a new folio, mlocks it with mlock_folio() and maps it in the page-table.
Since commit 2fbb0c10d1e8 ("mm/munlock: mlock_page() munlock_page() batch by pagevec"), mlock/munlock operations on a folio (formerly page), are deferred. For example, mlock_folio() takes an additional reference on the target folio before placing it into a per-cpu 'folio_batch' for later processing by mlock_folio_batch(), which drops the refcount once the operation is complete. Processing of the batches is coupled with the LRU batch logic and can be forcefully drained with lru_add_drain_all() but as long as a folio remains unprocessed on the batch, its refcount will be elevated.
This deferred batching therefore interacts poorly with the pKVM pinning scenario as we can find ourselves in a situation where the migration code fails to migrate a folio due to the elevated refcount from the pending mlock operation.
Hugh Dickins adds:-
!folio_test_lru() has never been a very reliable way to tell if an lru_add_drain_all() is worth calling, to remove LRU cache references to make the folio migratable: the LRU flag may be set even while the folio is held with an extra reference in a per-CPU LRU cache.
5.18 commit 2fbb0c10d1e8 may have made it more unreliable. Then 6.11 commit 33dfe9204f29 ("mm/gup: clear the LRU flag of a page before adding to LRU batch") tried to make it reliable, by moving LRU flag clearing; but missed the mlock/munlock batches, so still unreliable as reported.
And it turns out to be difficult to extend 33dfe9204f29's LRU flag clearing to the mlock/munlock batches: if they do benefit from batching, mlock/munlock cannot be so effective when easily suppressed while !LRU.
Instead, switch to an expected ref_count check, which was more reliable all along: some more false positives (unhelpful drains) than before, and never a guarantee that the folio will prove migratable, but better.
Note for stable backports: requires 6.16 commit 86ebd50224c0 ("mm: add folio_expected_ref_count() for reference count calculation") and 6.17 commit ("mm: fix folio_expected_ref_count() when PG_private_2").
Link: https://lkml.kernel.org/r/47c51c9a-140f-1ea1-b692-c4bae5d1fa58@google.com Fixes: 9a4e9f3b2d73 ("mm: update get_user_pages_longterm to migrate pages allocated from CMA region") Signed-off-by: Hugh Dickins hughd@google.com Reported-by: Will Deacon will@kernel.org Link: https://lore.kernel.org/linux-mm/20250815101858.24352-1-will@kernel.org/ Cc: stable@vger.kernel.org Cc: "Aneesh Kumar K.V" aneesh.kumar@kernel.org Cc: Axel Rasmussen axelrasmussen@google.com Cc: Chris Li chrisl@kernel.org Cc: Christoph Hellwig hch@infradead.org Cc: David Hildenbrand david@redhat.com Cc: Jason Gunthorpe jgg@ziepe.ca Cc: Johannes Weiner hannes@cmpxchg.org Cc: John Hubbard jhubbard@nvidia.com Cc: Keir Fraser keirf@google.com Cc: Konstantin Khlebnikov koct9i@gmail.com Cc: Li Zhe lizhe.67@bytedance.com Cc: Matthew Wilcox (Oracle) willy@infradead.org Cc: Peter Xu peterx@redhat.com Cc: Shivank Garg shivankg@amd.com Cc: Vlastimil Babka vbabka@suse.cz Cc: Wei Xu weixugc@google.com Cc: yangge yangge1116@126.com Cc: Yuanchu Xie yuanchu@google.com Cc: Yu Zhao yuzhao@google.com Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
mm/gup.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
--- a/mm/gup.c~mm-gup-check-ref_count-instead-of-lru-before-migration +++ a/mm/gup.c @@ -2307,7 +2307,8 @@ static unsigned long collect_longterm_un continue; }
- if (!folio_test_lru(folio) && drain_allow) { + if (drain_allow && folio_ref_count(folio) != + folio_expected_ref_count(folio) + 1) { lru_add_drain_all(); drain_allow = false; } _
Patches currently in -mm which might be from hughd@google.com are
mm-fix-folio_expected_ref_count-when-pg_private_2.patch mm-gup-check-ref_count-instead-of-lru-before-migration.patch mm-gup-local-lru_add_drain-to-avoid-lru_add_drain_all.patch mm-revert-mm-gup-clear-the-lru-flag-of-a-page-before-adding-to-lru-batch.patch mm-revert-mm-vmscanc-fix-oom-on-swap-stress-test.patch mm-folio_may_be_cached-unless-folio_test_large.patch mm-lru_add_drain_all-do-local-lru_add_drain-first.patch