[PATCH RFC 01/11] mm/migrate: extract folio unmap phase
From: Shivank Garg
Date: Wed Sep 02 2026 - 07:06:19 EST
migrate_pages_batch() was hard to follow with the unmap retry handling
inline alongside TLB flush, move, and cleanup. Extract the unmap phase
into migrate_folios_unmap(), which fills @unmap_folios and @dst_folios
for the steps that follow. This makes the unmap phase boundaries clear
and the code more readable.
No functional change intended.
Suggested-by: Zi Yan <ziy@xxxxxxxxxx>
Signed-off-by: Shivank Garg <shivankg@xxxxxxx>
---
mm/migrate.c | 89 ++++++++++++++++++++++++++++++++++++++++--------------------
1 file changed, 59 insertions(+), 30 deletions(-)
diff --git a/mm/migrate.c b/mm/migrate.c
index 15b45832bcfa..14a0f84009c6 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1800,36 +1800,30 @@ static void migrate_folios_undo(struct list_head *src_folios,
}
/*
- * migrate_pages_batch() first unmaps folios in the from list as many as
- * possible, then move the unmapped folios.
+ * Unmap the folios in @from, queuing successfully unmapped sources on
+ * @unmap_folios and their destination on @dst_folios. Increment @nr_failed for
+ * each failure.
*
- * We only batch migration if mode == MIGRATE_ASYNC to avoid to wait a
- * lock or bit when we have locked more than one folio. Which may cause
- * deadlock (e.g., for loop device). So, if mode != MIGRATE_ASYNC, the
- * length of the from list must be <= 1.
+ * Return: -ENOMEM on destination allocation failure stop the unmap phase,
+ * otherwise 0. Folios already unmapped remain queued for the move phase.
*/
-static int migrate_pages_batch(struct list_head *from,
+static int migrate_folios_unmap(struct list_head *from,
new_folio_t get_new_folio, free_folio_t put_new_folio,
unsigned long private, enum migrate_mode mode, enum migrate_reason reason,
struct list_head *ret_folios, struct list_head *split_folios,
- struct migrate_pages_stats *stats, int nr_pass)
+ struct list_head *unmap_folios, struct list_head *dst_folios,
+ struct migrate_pages_stats *stats, int nr_pass, int *nr_failed)
{
int retry = 1;
int thp_retry = 1;
- int nr_failed = 0;
int nr_retry_pages = 0;
int pass = 0;
bool is_thp = false;
bool is_large = false;
struct folio *folio, *folio2, *dst = NULL;
- int rc, rc_saved = 0, nr_pages;
- LIST_HEAD(unmap_folios);
- LIST_HEAD(dst_folios);
+ int rc, nr_pages;
bool nosplit = (reason == MR_NUMA_MISPLACED);
- VM_WARN_ON_ONCE(mode != MIGRATE_ASYNC &&
- !list_empty(from) && !list_is_singular(from));
-
for (pass = 0; pass < nr_pass && retry; pass++) {
retry = 0;
thp_retry = 0;
@@ -1869,7 +1863,7 @@ static int migrate_pages_batch(struct list_head *from,
!list_empty(&folio->_deferred_list) &&
folio_test_partially_mapped(folio)) {
if (!try_split_folio(folio, split_folios, mode)) {
- nr_failed++;
+ *nr_failed += 1;
stats->nr_thp_failed += is_thp;
stats->nr_thp_split += is_thp;
stats->nr_split++;
@@ -1888,7 +1882,7 @@ static int migrate_pages_batch(struct list_head *from,
* list is processed.
*/
if (!thp_migration_supported() && is_thp) {
- nr_failed++;
+ *nr_failed += 1;
stats->nr_thp_failed++;
if (!try_split_folio(folio, split_folios, mode)) {
stats->nr_thp_split++;
@@ -1925,13 +1919,13 @@ static int migrate_pages_batch(struct list_head *from,
* -ENOMEM: stay on the from list
* Other errno: put on ret_folios list
*/
- switch(rc) {
+ switch (rc) {
case -ENOMEM:
/*
* When memory is low, don't bother to try to migrate
* other folios, move unmapped folios, then exit.
*/
- nr_failed++;
+ *nr_failed += 1;
stats->nr_thp_failed += is_thp;
/* Large folio NUMA faulting doesn't split to retry. */
if (is_large && !nosplit) {
@@ -1951,7 +1945,7 @@ static int migrate_pages_batch(struct list_head *from,
thp_retry += is_thp;
nr_retry_pages += nr_pages;
/* Undo duplicated failure counting. */
- nr_failed--;
+ *nr_failed -= 1;
stats->nr_thp_failed -= is_thp;
break;
}
@@ -1960,19 +1954,15 @@ static int migrate_pages_batch(struct list_head *from,
stats->nr_failed_pages += nr_pages + nr_retry_pages;
/* nr_failed isn't updated for not used */
stats->nr_thp_failed += thp_retry;
- rc_saved = rc;
- if (list_empty(&unmap_folios))
- goto out;
- else
- goto move;
+ return -ENOMEM;
case -EAGAIN:
retry++;
thp_retry += is_thp;
nr_retry_pages += nr_pages;
break;
case 0:
- list_move_tail(&folio->lru, &unmap_folios);
- list_add_tail(&dst->lru, &dst_folios);
+ list_move_tail(&folio->lru, unmap_folios);
+ list_add_tail(&dst->lru, dst_folios);
break;
default:
/*
@@ -1981,17 +1971,56 @@ static int migrate_pages_batch(struct list_head *from,
* removed from migration folio list and not
* retried in the next outer loop.
*/
- nr_failed++;
+ *nr_failed += 1;
stats->nr_thp_failed += is_thp;
stats->nr_failed_pages += nr_pages;
break;
}
}
}
- nr_failed += retry;
+ *nr_failed += retry;
stats->nr_thp_failed += thp_retry;
stats->nr_failed_pages += nr_retry_pages;
-move:
+
+ return 0;
+}
+
+/*
+ * migrate_pages_batch() first unmaps as many folios in the source list as
+ * possible, flushes the TLBs, then moves the unmapped folios.
+ *
+ * Only MIGRATE_ASYNC may batch multiple folios. Waiting for a lock or bit
+ * while multiple folios are locked may deadlock (e.g. with a loop device).
+ * Therefore, if mode != MIGRATE_ASYNC, the source list must contain at most
+ * one folio.
+ */
+static int migrate_pages_batch(struct list_head *from,
+ new_folio_t get_new_folio, free_folio_t put_new_folio,
+ unsigned long private, enum migrate_mode mode, enum migrate_reason reason,
+ struct list_head *ret_folios, struct list_head *split_folios,
+ struct migrate_pages_stats *stats, int nr_pass)
+{
+ int retry = 1;
+ int thp_retry = 1;
+ int nr_failed = 0;
+ int nr_retry_pages = 0;
+ int pass = 0;
+ int rc, rc_saved;
+ LIST_HEAD(unmap_folios);
+ LIST_HEAD(dst_folios);
+
+ VM_WARN_ON_ONCE(mode != MIGRATE_ASYNC &&
+ !list_empty(from) && !list_is_singular(from));
+
+ rc_saved = migrate_folios_unmap(from, get_new_folio, put_new_folio,
+ private, mode, reason, ret_folios, split_folios,
+ &unmap_folios, &dst_folios, stats, nr_pass,
+ &nr_failed);
+ if (rc_saved && list_empty(&unmap_folios)) {
+ rc = rc_saved;
+ goto out;
+ }
+
/* Flush TLBs for all unmapped folios */
try_to_unmap_flush();
--
2.43.0