* [merged mm-hotfixes-stable] mm-fix-zswap-writeback-race-condition.patch removed from -mm tree
@ 2023-05-17 22:25 Andrew Morton
0 siblings, 0 replies; only message in thread
From: Andrew Morton @ 2023-05-17 22:25 UTC (permalink / raw
To: mm-commits, vitaly.wool, stable, sjenning, ngupta, minchan,
hannes, ddstreet, chrisl, cerasuolodomenico, akpm
The quilt patch titled
Subject: mm: fix zswap writeback race condition
has been removed from the -mm tree. Its filename was
mm-fix-zswap-writeback-race-condition.patch
This patch was dropped because it was merged into the mm-hotfixes-stable branch
of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
------------------------------------------------------
From: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
Subject: mm: fix zswap writeback race condition
Date: Wed, 3 May 2023 17:12:00 +0200
The zswap writeback mechanism can cause a race condition resulting in
memory corruption, where a swapped out page gets swapped in with data that
was written to a different page.
The race unfolds like this:
1. a page with data A and swap offset X is stored in zswap
2. page A is removed off the LRU by zpool driver for writeback in
zswap-shrink work, data for A is mapped by zpool driver
3. user space program faults and invalidates page entry A, offset X is
considered free
4. kswapd stores page B at offset X in zswap (zswap could also be
full, if so, page B would then be IOed to X, then skip step 5.)
5. entry A is replaced by B in tree->rbroot, this doesn't affect the
local reference held by zswap-shrink work
6. zswap-shrink work writes back A at X, and frees zswap entry A
7. swapin of slot X brings A in memory instead of B
The fix:
Once the swap page cache has been allocated (case ZSWAP_SWAPCACHE_NEW),
zswap-shrink work just checks that the local zswap_entry reference is
still the same as the one in the tree. If it's not the same it means that
it's either been invalidated or replaced, in both cases the writeback is
aborted because the local entry contains stale data.
Reproducer:
I originally found this by running `stress` overnight to validate my work
on the zswap writeback mechanism, it manifested after hours on my test
machine. The key to make it happen is having zswap writebacks, so
whatever setup pumps /sys/kernel/debug/zswap/written_back_pages should do
the trick.
In order to reproduce this faster on a vm, I setup a system with ~100M of
available memory and a 500M swap file, then running `stress --vm 1
--vm-bytes 300000000 --vm-stride 4000` makes it happen in matter of tens
of minutes. One can speed things up even more by swinging
/sys/module/zswap/parameters/max_pool_percent up and down between, say, 20
and 1; this makes it reproduce in tens of seconds. It's crucial to set
`--vm-stride` to something other than 4096 otherwise `stress` won't
realize that memory has been corrupted because all pages would have the
same data.
Link: https://lkml.kernel.org/r/20230503151200.19707-1-cerasuolodomenico@gmail.com
Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@gmail.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Chris Li (Google) <chrisl@kernel.org>
Cc: Dan Streetman <ddstreet@ieee.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nitin Gupta <ngupta@vflare.org>
Cc: Seth Jennings <sjenning@redhat.com>
Cc: Vitaly Wool <vitaly.wool@konsulko.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
mm/zswap.c | 16 ++++++++++++++++
1 file changed, 16 insertions(+)
--- a/mm/zswap.c~mm-fix-zswap-writeback-race-condition
+++ a/mm/zswap.c
@@ -1020,6 +1020,22 @@ static int zswap_writeback_entry(struct
goto fail;
case ZSWAP_SWAPCACHE_NEW: /* page is locked */
+ /*
+ * Having a local reference to the zswap entry doesn't exclude
+ * swapping from invalidating and recycling the swap slot. Once
+ * the swapcache is secured against concurrent swapping to and
+ * from the slot, recheck that the entry is still current before
+ * writing.
+ */
+ spin_lock(&tree->lock);
+ if (zswap_rb_search(&tree->rbroot, entry->offset) != entry) {
+ spin_unlock(&tree->lock);
+ delete_from_swap_cache(page_folio(page));
+ ret = -ENOMEM;
+ goto fail;
+ }
+ spin_unlock(&tree->lock);
+
/* decompress */
acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);
dlen = PAGE_SIZE;
_
Patches currently in -mm which might be from cerasuolodomenico@gmail.com are
^ permalink raw reply [flat|nested] only message in thread
only message in thread, other threads:[~2023-05-17 22:25 UTC | newest]
Thread overview: (only message) (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2023-05-17 22:25 [merged mm-hotfixes-stable] mm-fix-zswap-writeback-race-condition.patch removed from -mm tree Andrew Morton
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).