The patch titled
Subject: mm: fix crashes from deferred split racing folio migration
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
mm-fix-crashes-from-deferred-split-racing-folio-migration.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-fix-crashes-from-deferred-split-racing-folio-migration.patch
This patch will later appear in the mm-hotfixes-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Hugh Dickins <hughd@xxxxxxxxxx>
Subject: mm: fix crashes from deferred split racing folio migration
Date: Tue, 2 Jul 2024 00:40:55 -0700 (PDT)
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/29c83d1a-11ca-b6c9-f92e-6ccb322af510@xxxxxxxxxx
Fixes: 9bcef5973e31 ("mm: memcg: fix split queue list crash when large folio migration")
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Reviewed-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Cc: Barry Song <baohua@xxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Cc: Nhat Pham <nphamcs@xxxxxxxxx>
Cc: Yang Shi <shy828301@xxxxxxxxx>
Cc: Zi Yan <ziy@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/memcontrol.c | 11 -----------
mm/migrate.c | 13 +++++++++++++
2 files changed, 13 insertions(+), 11 deletions(-)
--- a/mm/memcontrol.c~mm-fix-crashes-from-deferred-split-racing-folio-migration
+++ a/mm/memcontrol.c
@@ -7823,17 +7823,6 @@ void mem_cgroup_migrate(struct folio *ol
/* 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;
}
--- a/mm/migrate.c~mm-fix-crashes-from-deferred-split-racing-folio-migration
+++ a/mm/migrate.c
@@ -415,6 +415,15 @@ int folio_migrate_mapping(struct address
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
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.
_
Patches currently in -mm which might be from hughd@xxxxxxxxxx are
mm-fix-crashes-from-deferred-split-racing-folio-migration.patch
