From: Jan Kara <jack@xxxxxxx>
Subject: mm: migrate: fix reference check race between __find_get_block() and migration
buffer_migrate_page_norefs() can race with bh users in the following way:
CPU1 CPU2
buffer_migrate_page_norefs()
buffer_migrate_lock_buffers()
checks bh refs
spin_unlock(&mapping->private_lock)
__find_get_block()
spin_lock(&mapping->private_lock)
grab bh ref
spin_unlock(&mapping->private_lock)
move page do bh work
This can result in various issues like lost updates to buffers (i.e.
metadata corruption) or use after free issues for the old page.
This patch closes the race by holding mapping->private_lock while the
mapping is being moved to a new page. Ordinarily, a reference can be
taken outside of the private_lock using the per-cpu BH LRU but the
references are checked and the LRU invalidated if necessary. The
private_lock is held once the references are known so the buffer lookup
slow path will spin on the private_lock. Between the page lock and
private_lock, it should be impossible for other references to be acquired
and updates to happen during the migration.
A user had reported data corruption issues on a distribution kernel with a
similar page migration implementation as mainline. The data corruption
could not be reproduced with this patch applied. A small number of
migration-intensive tests were run and no performance problems were noted.
[mgorman@xxxxxxxxxxxxxxxxxxx: Changelog, removed tracing]
Link: http://lkml.kernel.org/r/20190718090238.GF24383@xxxxxxxxxxxxxxxxxxx
Fixes: 89cb0888ca14 "mm: migrate: provide buffer_migrate_page_norefs()"
Signed-off-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> [5.0+]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/migrate.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
--- a/mm/migrate.c~mm-migrate-fix-reference-check-race-between-__find_get_block-and-migration
+++ a/mm/migrate.c
@@ -767,12 +767,12 @@ recheck_buffers:
}
bh = bh->b_this_page;
} while (bh != head);
- spin_unlock(&mapping->private_lock);
if (busy) {
if (invalidated) {
rc = -EAGAIN;
goto unlock_buffers;
}
+ spin_unlock(&mapping->private_lock);
invalidate_bh_lrus();
invalidated = true;
goto recheck_buffers;
@@ -805,6 +805,8 @@ recheck_buffers:
rc = MIGRATEPAGE_SUCCESS;
unlock_buffers:
+ if (check_refs)
+ spin_unlock(&mapping->private_lock);
bh = head;
do {
unlock_buffer(bh);
_
