Commit be9581ea authored by Hugh Dickins's avatar Hugh Dickins Committed by Andrew Morton
Browse files

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 9bcef597 ("mm: memcg: fix split queue list crash when large
folio migration") was right to fix the memcg-dependent locking broken in
85ce2c51 ("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/29c83d1a-11ca-b6c9-f92e-6ccb322af510@google.com


Fixes: 9bcef597 ("mm: memcg: fix split queue list crash when large folio migration")
Signed-off-by: default avatarHugh Dickins <hughd@google.com>
Reviewed-by: default avatarBaolin Wang <baolin.wang@linux.alibaba.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 2fe29fe9

mm/memcontrol.c

+0 −11
Original line number Diff line number Diff line
@@ -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;

}

mm/migrate.c

+13 −0
Original line number Diff line number Diff line
@@ -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.