6.8-stable review patch. If anyone has any objections, please let me know.
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From: Johannes Weiner hannes@cmpxchg.org
commit 682886ec69d22363819a83ddddd5d66cb5c791e1 upstream.
Christian reports a NULL deref in zswap that he bisected down to the zswap shrinker. The issue also cropped up in the bug trackers of libguestfs [1] and the Red Hat bugzilla [2].
The problem is that when memcg is disabled with the boot time flag, the zswap shrinker might get called with sc->memcg == NULL. This is okay in many places, like the lruvec operations. But it crashes in memcg_page_state() - which is only used due to the non-node accounting of cgroup's the zswap memory to begin with.
Nhat spotted that the memcg can be NULL in the memcg-disabled case, and I was then able to reproduce the crash locally as well.
[1] https://github.com/libguestfs/libguestfs/issues/139 [2] https://bugzilla.redhat.com/show_bug.cgi?id=2275252
Link: https://lkml.kernel.org/r/20240418124043.GC1055428@cmpxchg.org Link: https://lkml.kernel.org/r/20240417143324.GA1055428@cmpxchg.org Fixes: b5ba474f3f51 ("zswap: shrink zswap pool based on memory pressure") Signed-off-by: Johannes Weiner hannes@cmpxchg.org Reported-by: Christian Heusel christian@heusel.eu Debugged-by: Nhat Pham nphamcs@gmail.com Suggested-by: Nhat Pham nphamcs@gmail.com Tested-by: Christian Heusel christian@heusel.eu Acked-by: Yosry Ahmed yosryahmed@google.com Cc: Chengming Zhou chengming.zhou@linux.dev Cc: Dan Streetman ddstreet@ieee.org Cc: Richard W.M. Jones rjones@redhat.com Cc: Seth Jennings sjenning@redhat.com Cc: Vitaly Wool vitaly.wool@konsulko.com Cc: stable@vger.kernel.org [v6.8] Signed-off-by: Andrew Morton akpm@linux-foundation.org Signed-off-by: Johannes Weiner hannes@cmpxchg.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- mm/zswap.c | 25 ++++++++++++++++--------- 1 file changed, 16 insertions(+), 9 deletions(-)
Two minor conflicts in the else branch: - zswap_pool_total_size was get_zswap_pool_size() in 6.8 - zswap_nr_stored was pool->nr_stored in 6.8
--- a/mm/zswap.c +++ b/mm/zswap.c @@ -653,15 +653,22 @@ static unsigned long zswap_shrinker_coun if (!gfp_has_io_fs(sc->gfp_mask)) return 0;
-#ifdef CONFIG_MEMCG_KMEM - mem_cgroup_flush_stats(memcg); - nr_backing = memcg_page_state(memcg, MEMCG_ZSWAP_B) >> PAGE_SHIFT; - nr_stored = memcg_page_state(memcg, MEMCG_ZSWAPPED); -#else - /* use pool stats instead of memcg stats */ - nr_backing = get_zswap_pool_size(pool) >> PAGE_SHIFT; - nr_stored = atomic_read(&pool->nr_stored); -#endif + /* + * For memcg, use the cgroup-wide ZSWAP stats since we don't + * have them per-node and thus per-lruvec. Careful if memcg is + * runtime-disabled: we can get sc->memcg == NULL, which is ok + * for the lruvec, but not for memcg_page_state(). + * + * Without memcg, use the zswap pool-wide metrics. + */ + if (!mem_cgroup_disabled()) { + mem_cgroup_flush_stats(memcg); + nr_backing = memcg_page_state(memcg, MEMCG_ZSWAP_B) >> PAGE_SHIFT; + nr_stored = memcg_page_state(memcg, MEMCG_ZSWAPPED); + } else { + nr_backing = get_zswap_pool_size(pool) >> PAGE_SHIFT; + nr_stored = atomic_read(&pool->nr_stored); + }
if (!nr_stored) return 0;