mm/migrate: correct nr_failed in migrate_pages_sync()

nr_failed was missing the large folio splits from migrate_pages_batch()
and can cause a mismatch between migrate_pages() return value and the
number of not migrated pages, i.e., when the return value of
migrate_pages() is 0, there are still pages left in the from page list.
It will happen when a non-PMD THP large folio fails to migrate due to
-ENOMEM and is split successfully but not all the split pages are not
migrated, migrate_pages_batch() would return non-zero, but
astats.nr_thp_split = 0.  nr_failed would be 0 and returned to the caller
of migrate_pages(), but the not migrated pages are left in the from page
list without being added back to LRU lists.

Fix it by adding a new nr_split counter for large folio splits and adding
it to nr_failed in migrate_page_sync() after migrate_pages_batch() is
done.

Link: https://lkml.kernel.org/r/20231017163129.2025214-1-zi.yan@sent.com
Fixes: 2ef7dbb26990 ("migrate_pages: try migrate in batch asynchronously firstly")
Signed-off-by: Zi Yan <ziy@nvidia.com>
Acked-by: Huang Ying <ying.huang@intel.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/migrate.c b/mm/migrate.c
index 71fc5064cec669a628529a3cdd94d2be585b537f..53244d283c3e12754852c07144826c25b04fac8d 100644 (file)
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1495,6 +1495,7 @@ struct migrate_pages_stats {
        int nr_thp_succeeded;   /* THP migrated successfully */
        int nr_thp_failed;      /* THP failed to be migrated */
        int nr_thp_split;       /* THP split before migrating */
+       int nr_split;   /* Large folio (include THP) split before migrating */
 };
 
 /*
@@ -1614,6 +1615,7 @@ static int migrate_pages_batch(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, *dst2;
        int rc, rc_saved = 0, nr_pages;
        LIST_HEAD(unmap_folios);
@@ -1629,7 +1631,8 @@ static int migrate_pages_batch(struct list_head *from,
                nr_retry_pages = 0;
 
                list_for_each_entry_safe(folio, folio2, from, lru) {
-                       is_thp = folio_test_large(folio) && folio_test_pmd_mappable(folio);
+                       is_large = folio_test_large(folio);
+                       is_thp = is_large && folio_test_pmd_mappable(folio);
                        nr_pages = folio_nr_pages(folio);
 
                        cond_resched();
@@ -1649,6 +1652,7 @@ static int migrate_pages_batch(struct list_head *from,
                                stats->nr_thp_failed++;
                                if (!try_split_folio(folio, split_folios)) {
                                        stats->nr_thp_split++;
+                                       stats->nr_split++;
                                        continue;
                                }
                                stats->nr_failed_pages += nr_pages;
@@ -1677,11 +1681,12 @@ static int migrate_pages_batch(struct list_head *from,
                                nr_failed++;
                                stats->nr_thp_failed += is_thp;
                                /* Large folio NUMA faulting doesn't split to retry. */
-                               if (folio_test_large(folio) && !nosplit) {
+                               if (is_large && !nosplit) {
                                        int ret = try_split_folio(folio, split_folios);
 
                                        if (!ret) {
                                                stats->nr_thp_split += is_thp;
+                                               stats->nr_split += is_large;
                                                break;
                                        } else if (reason == MR_LONGTERM_PIN &&
                                                   ret == -EAGAIN) {
@@ -1827,6 +1832,7 @@ static int migrate_pages_sync(struct list_head *from, new_folio_t get_new_folio,
        stats->nr_succeeded += astats.nr_succeeded;
        stats->nr_thp_succeeded += astats.nr_thp_succeeded;
        stats->nr_thp_split += astats.nr_thp_split;
+       stats->nr_split += astats.nr_split;
        if (rc < 0) {
                stats->nr_failed_pages += astats.nr_failed_pages;
                stats->nr_thp_failed += astats.nr_thp_failed;
@@ -1834,7 +1840,11 @@ static int migrate_pages_sync(struct list_head *from, new_folio_t get_new_folio,
                return rc;
        }
        stats->nr_thp_failed += astats.nr_thp_split;
-       nr_failed += astats.nr_thp_split;
+       /*
+        * Do not count rc, as pages will be retried below.
+        * Count nr_split only, since it includes nr_thp_split.
+        */
+       nr_failed += astats.nr_split;
        /*
         * Fall back to migrate all failed folios one by one synchronously. All
         * failed folios except split THPs will be retried, so their failure