[PATCH RFC 03/11] mm/migrate: factor out folio splitting on allocation failure

[Shivank Garg]

If destination folio allocation fails, migrate_folios_unmap() may split
a large folio and retry migration with smaller folios. Factor this logic
out into migrate_folio_split_on_alloc_fail() to simplify the allocation
failure path.

No functional change intended.

Signed-off-by: Shivank Garg <shivankg@xxxxxxx>
---
 mm/migrate.c | 69 +++++++++++++++++++++++++++++++++++++-----------------------
 1 file changed, 43 insertions(+), 26 deletions(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index 89207bf515b9..510dfffa7235 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1809,6 +1809,34 @@ static void migrate_folios_undo(struct list_head *src_folios,
 	}
 }
 
+static int migrate_folio_split_on_alloc_fail(struct folio *folio,
+		struct list_head *split_folios, enum migrate_mode mode,
+		enum migrate_reason reason, struct migrate_pages_stats *stats)
+{
+	bool is_thp = folio_test_pmd_mappable(folio);
+	int rc;
+
+	/* Large folio NUMA faulting doesn't split to retry. */
+	if (!folio_test_large(folio) || reason == MR_NUMA_MISPLACED)
+		return -ENOMEM;
+
+	rc = try_split_folio(folio, split_folios, mode);
+	if (!rc) {
+		stats->nr_thp_split += is_thp;
+		stats->nr_split++;
+		return 0;
+	}
+
+	/*
+	 * Try again to split large folio to mitigate the failure of longterm
+	 * pinning.
+	 */
+	if (reason == MR_LONGTERM_PIN && rc == -EAGAIN)
+		return -EAGAIN;
+
+	return -ENOMEM;
+}
+
 /*
  * Unmap the folios in @from, queuing successfully unmapped sources on
  * @unmap_folios and their destination on @dst_folios.  Increment @nr_failed for
@@ -1829,10 +1857,8 @@ static int migrate_folios_unmap(struct list_head *from,
 	int nr_retry_pages = 0;
 	int pass = 0;
 	bool is_thp = false;
-	bool is_large = false;
 	struct folio *folio, *folio2, *dst = NULL;
-	int rc, nr_pages;
-	bool nosplit = (reason == MR_NUMA_MISPLACED);
+	int rc, split_rc, nr_pages;
 
 	for (pass = 0; pass < nr_pass && retry; pass++) {
 		retry = 0;
@@ -1840,7 +1866,6 @@ static int migrate_folios_unmap(struct list_head *from,
 		nr_retry_pages = 0;
 
 		list_for_each_entry_safe(folio, folio2, from, lru) {
-			is_large = folio_test_large(folio);
 			is_thp = folio_test_pmd_mappable(folio);
 			nr_pages = folio_nr_pages(folio);
 
@@ -1937,28 +1962,20 @@ static int migrate_folios_unmap(struct list_head *from,
 				 */
 				*nr_failed += 1;
 				stats->nr_thp_failed += is_thp;
-				/* Large folio NUMA faulting doesn't split to retry. */
-				if (is_large && !nosplit) {
-					int ret = try_split_folio(folio, split_folios, mode);
-
-					if (!ret) {
-						stats->nr_thp_split += is_thp;
-						stats->nr_split++;
-						break;
-					} else if (reason == MR_LONGTERM_PIN &&
-						   ret == -EAGAIN) {
-						/*
-						 * Try again to split large folio to
-						 * mitigate the failure of longterm pinning.
-						 */
-						retry++;
-						thp_retry += is_thp;
-						nr_retry_pages += nr_pages;
-						/* Undo duplicated failure counting. */
-						*nr_failed -= 1;
-						stats->nr_thp_failed -= is_thp;
-						break;
-					}
+
+				split_rc = migrate_folio_split_on_alloc_fail(folio,
+						split_folios, mode, reason, stats);
+				if (!split_rc)
+					break;
+				/* Retry this folio in a later pass. */
+				if (split_rc == -EAGAIN) {
+					retry++;
+					thp_retry += is_thp;
+					nr_retry_pages += nr_pages;
+					/* Undo duplicated failure counting. */
+					*nr_failed -= 1;
+					stats->nr_thp_failed -= is_thp;
+					break;
 				}
 
 				stats->nr_failed_pages += nr_pages + nr_retry_pages;

-- 
2.43.0