mm: fix crashes from deferred split racing folio migration

Even on 6.10-rc6, I've been seeing elusive "Bad page state"s (often on
flags when freeing, yet the flags shown are not bad: PG_locked had been
set and cleared??), and VM_BUG_ON_PAGE(page_ref_count(page) == 0)s from
deferred_split_scan()'s folio_put(), and a variety of other BUG and WARN
symptoms implying double free by deferred split and large folio migration.

6.7 commit 9bcef5973e31 ("mm: memcg: fix split queue list crash when large
folio migration") was right to fix the memcg-dependent locking broken in
85ce2c517ade ("memcontrol: only transfer the memcg data for migration"),
but missed a subtlety of deferred_split_scan(): it moves folios to its own
local list to work on them without split_queue_lock, during which time
folio->_deferred_list is not empty, but even the "right" lock does nothing
to secure the folio and the list it is on.

Fortunately, deferred_split_scan() is careful to use folio_try_get(): so
folio_migrate_mapping() can avoid the race by folio_undo_large_rmappable()
while the old folio's reference count is temporarily frozen to 0 - adding
such a freeze in the !mapping case too (originally, folio lock and
unmapping and no swap cache left an anon folio unreachable, so no freezing
was needed there: but the deferred split queue offers a way to reach it).

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 9bcef5973e31 ("mm: memcg: fix split queue list crash when large folio migration")
Signed-off-by: Hugh Dickins <[email protected]>
Reviewed-by: Baolin Wang <[email protected]>
Cc: Barry Song <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Kefeng Wang <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Nhat Pham <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: Zi Yan <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 71fe2a95b8bd33fdc0b1ff3fec3e3eef74eff4b7..8f2f1bb18c9cb8775ce771dd81159d724cd2b911 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -7823,17 +7823,6 @@ void mem_cgroup_migrate(struct folio *old, struct folio *new)
 
        /* Transfer the charge and the css ref */
        commit_charge(new, memcg);
-       /*
-        * If the old folio is a large folio and is in the split queue, it needs
-        * to be removed from the split queue now, in case getting an incorrect
-        * split queue in destroy_large_folio() after the memcg of the old folio
-        * is cleared.
-        *
-        * In addition, the old folio is about to be freed after migration, so
-        * removing from the split queue a bit earlier seems reasonable.
-        */
-       if (folio_test_large(old) && folio_test_large_rmappable(old))
-               folio_undo_large_rmappable(old);
        old->memcg_data = 0;
 }
 

diff --git a/mm/migrate.c b/mm/migrate.c
index 20cb9f5f74460518881617788dcf28843a45a026..a8c6f466e33acc98faea26b52cebc143d7153949 100644 (file)
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -415,6 +415,15 @@ int folio_migrate_mapping(struct address_space *mapping,
                if (folio_ref_count(folio) != expected_count)
                        return -EAGAIN;
 
+               /* Take off deferred split queue while frozen and memcg set */
+               if (folio_test_large(folio) &&
+                   folio_test_large_rmappable(folio)) {
+                       if (!folio_ref_freeze(folio, expected_count))
+                               return -EAGAIN;
+                       folio_undo_large_rmappable(folio);
+                       folio_ref_unfreeze(folio, expected_count);
+               }
+
                /* No turning back from here */
                newfolio->index = folio->index;
                newfolio->mapping = folio->mapping;
@@ -433,6 +442,10 @@ int folio_migrate_mapping(struct address_space *mapping,
                return -EAGAIN;
        }
 
+       /* Take off deferred split queue while frozen and memcg set */
+       if (folio_test_large(folio) && folio_test_large_rmappable(folio))
+               folio_undo_large_rmappable(folio);
+
        /*
         * Now we know that no one else is looking at the folio:
         * no turning back from here.