+ mm-fix-crashes-from-deferred-split-racing-folio-migration.patch added to mm-hotfixes-unstable branch

+ mm-fix-crashes-from-deferred-split-racing-folio-migration.patch added to mm-hotfixes-unstable branch

[Andrew Morton]

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@google.com>
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@google.com
Fixes: 9bcef5973e31 ("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>
---

 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@google.com are

mm-fix-crashes-from-deferred-split-racing-folio-migration.patch