commit 98c6d259319ecf6e8d027abd3f14b81324b8c0ad upstream.
Patch series "mm: better GUP pin lru_add_drain_all()", v2.
Series of lru_add_drain_all()-related patches, arising from recent mm/gup
migration report from Will Deacon.
This patch (of 5):
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 2fbb0c10d1 ("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 2fbb0c10d1 may have made it more unreliable. Then 6.11
commit 33dfe9204f ("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 on PG_private_2: ceph and nfs are still using the deprecated
PG_private_2 flag, with the aid of netfs and filemap support functions.
Although it is consistently matched by an increment of folio ref_count,
folio_expected_ref_count() intentionally does not recognize it, and ceph
folio migration currently depends on that for PG_private_2 folios to be
rejected. New references to the deprecated flag are discouraged, so do
not add it into the collect_longterm_unpinnable_folios() calculation: but
longterm pinning of transiently PG_private_2 ceph and nfs folios (an
uncommon case) may invoke a redundant lru_add_drain_all(). And this makes
easy the backport to earlier releases: up to and including 6.12, btrfs
also used PG_private_2, but without a ref_count increment.
Note for stable backports: requires 6.16 commit 86ebd50224 ("mm:
add folio_expected_ref_count() for reference count calculation").
Link: https://lkml.kernel.org/r/41395944-b0e3-c3ac-d648-8ddd70451d28@google.com
Link: https://lkml.kernel.org/r/bd1f314a-fca1-8f19-cac0-b936c9614557@google.com
Fixes: 9a4e9f3b2d ("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>
Closes: https://lore.kernel.org/linux-mm/20250815101858.24352-1-will@kernel.org/
Acked-by: Kiryl Shutsemau <kas@kernel.org>
Acked-by: David Hildenbrand <david@redhat.com>
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: 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: Rik van Riel <riel@surriel.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>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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