[PATCH hotfix] mm: fix crashes from deferred split racing folio migration

Hugh Dickins
2024-07-02T07:41:14Z

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).

Fixes: 9bcef5973e31 ("mm: memcg: fix split queue list crash when large folio migration")
Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: stable@vger.kernel.org
---
This patch against 6.10-rc6: Kefeng has commits in the mm-tree which
which will need adjustment to go over this, but we can both check the
result. I have wondered whether just reverting 85ce2c517ade and its
subsequent fixups would be better: but that would be a bigger job,
and probably not the right choice.

mm/memcontrol.c | 11 -----------
mm/migrate.c | 13 +++++++++++++
2 files changed, 13 insertions(+), 11 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 71fe2a95b8bd..8f2f1bb18c9c 100644
--- 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 20cb9f5f7446..a8c6f466e33a 100644
--- 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.
--
2.35.3