[PATCH] 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>
---

This patch was part of
https://lore.kernel.org/all/20250625055806.82645-1-dev.jain@arm.com/
but I have sent it separately on suggestion of Lorenzo, and also because
I plan to send the first two patches after David Hildenbrand's
folio_pte_batch series gets merged.

 mm/khugepaged.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 1aa7ca67c756..99977bb9bf6a 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -31,6 +31,7 @@ enum scan_result {
 	SCAN_FAIL,
 	SCAN_SUCCEED,
 	SCAN_PMD_NULL,
+	SCAN_PMD_MIGRATION,
 	SCAN_PMD_NONE,
 	SCAN_PMD_MAPPED,
 	SCAN_EXCEED_NONE_PTE,
@@ -941,6 +942,8 @@ static inline int check_pmd_state(pmd_t *pmd)
 
 	if (pmd_none(pmde))
 		return SCAN_PMD_NONE;
+	if (is_pmd_migration_entry(pmde))
+		return SCAN_PMD_MIGRATION;
 	if (!pmd_present(pmde))
 		return SCAN_PMD_NULL;
 	if (pmd_trans_huge(pmde))
@@ -1502,9 +1505,12 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr,
 	    !range_in_vma(vma, haddr, haddr + HPAGE_PMD_SIZE))
 		return SCAN_VMA_CHECK;
 
-	/* Fast check before locking page if already PMD-mapped */
+	/*
+	 * Fast check before locking folio if already PMD-mapped, or if the
+	 * folio is under migration
+	 */
 	result = find_pmd_or_thp_or_none(mm, haddr, &pmd);
-	if (result == SCAN_PMD_MAPPED)
+	if (result == SCAN_PMD_MAPPED || result == SCAN_PMD_MIGRATION)
 		return result;
 
 	/*
@@ -2716,6 +2722,7 @@ static int madvise_collapse_errno(enum scan_result r)
 	case SCAN_PAGE_LRU:
 	case SCAN_DEL_PAGE_LRU:
 	case SCAN_PAGE_FILLED:
+	case SCAN_PMD_MIGRATION:
 		return -EAGAIN;
 	/*
 	 * Other: Trying again likely not to succeed / error intrinsic to
@@ -2802,6 +2809,7 @@ int madvise_collapse(struct vm_area_struct *vma, unsigned long start,
 			goto handle_result;
 		/* Whitelisted set of results where continuing OK */
 		case SCAN_PMD_NULL:
+		case SCAN_PMD_MIGRATION:
 		case SCAN_PTE_NON_PRESENT:
 		case SCAN_PTE_UFFD_WP:
 		case SCAN_PAGE_RO:
-- 
2.30.2