[PATCH v2] khugepaged: Reduce race probability between migration and khugepaged

[Dev Jain <dev.jain@arm.com>]

Suppose a folio is under migration, and khugepaged is also trying to
collapse it. collapse_pte_mapped_thp() will retrieve the folio from the
page cache via filemap_lock_folio(), thus taking a reference on the folio
and sleeping on the folio lock, since the lock is held by the migration
path. Migration will then fail in
__folio_migrate_mapping -> folio_ref_freeze. Reduce the probability of
such a race happening (leading to migration failure) by bailing out
if we detect a PMD is marked with a migration entry.

This fixes the migration-shared-anon-thp testcase failure on Apple M3.

Note that, this is not a "fix" since it only reduces the chance of
interference of khugepaged with migration, wherein both the kernel
functionalities are deemed "best-effort".

Signed-off-by: Dev Jain <dev.jain@arm.com>
---

v1->v2:
 - Remove SCAN_PMD_MIGRATION, merge into SCAN_PMD_MAPPED (David, Anshuman)
 - Add a comment (Lorenzo)

v1:
 - https://lore.kernel.org/all/20250630044837.4675-1-dev.jain@arm.com/

 mm/khugepaged.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 1aa7ca67c756..3fdefc4f4984 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -941,6 +941,15 @@ static inline int check_pmd_state(pmd_t *pmd)
 
 	if (pmd_none(pmde))
 		return SCAN_PMD_NONE;
+
+	/*
+	 * The folio may be under migration when khugepaged is trying to
+	 * collapse it. Migration success or failure will eventually end
+	 * up with the PMD still pointing to a PMD-order folio, so return
+	 * SCAN_PMD_MAPPED.
+	 */
+	if (is_pmd_migration_entry(pmde))
+		return SCAN_PMD_MAPPED;
 	if (!pmd_present(pmde))
 		return SCAN_PMD_NULL;
 	if (pmd_trans_huge(pmde))
-- 
2.30.2