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mm: fix crashes from deferred split racing folio migration

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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 9bcef5973e ("mm: memcg: fix split queue list crash when large
folio migration") was right to fix the memcg-dependent locking broken in
85ce2c517a ("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: 9bcef5973e ("mm: memcg: fix split queue list crash when large folio migration")
Signed-off-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Baolin 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: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Hugh Dickins 2024-07-02 00:40:55 -07:00 committed by Andrew Morton
parent 2fe29fe945
commit be9581ea8c
2 changed files with 13 additions and 11 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

11
mm/memcontrol.c
View file

@ -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;
}

13
mm/migrate.c
View file

@ -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.