FAILED: patch "[PATCH] mm/gup: check ref_count instead of lru before migration" failed to apply to 5.10-stable tree

The patch below does not apply to the 5.10-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.10.y
git checkout FETCH_HEAD
git cherry-pick -x 98c6d259319ecf6e8d027abd3f14b81324b8c0ad
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2025092154-canon-user-98b7@gregkh' --subject-prefix 'PATCH 5.10.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 98c6d259319ecf6e8d027abd3f14b81324b8c0ad Mon Sep 17 00:00:00 2001
From: Hugh Dickins <[email protected]>
Date: Mon, 8 Sep 2025 15:15:03 -0700
Subject: [PATCH] mm/gup: check ref_count instead of lru before migration

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 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 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 86ebd50224c0 ("mm:
add folio_expected_ref_count() for reference count calculation").

index adffe663594d..82aec6443c0a 100644
@@ -2307,7 +2307,8 @@ static unsigned long collect_longterm_unpinnable_folios(
 			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;
 		}