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@xxxxxxxxxxx Link: https://lkml.kernel.org/r/20240417143324.GA1055428@xxxxxxxxxxx Fixes: b5ba474f3f51 ("zswap: shrink zswap pool based on memory pressure") Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx> Reported-by: Christian Heusel <christian@xxxxxxxxx> Debugged-by: Nhat Pham <nphamcs@xxxxxxxxx> Suggested-by: Nhat Pham <nphamcs@xxxxxxxxx> Tested-by: Christian Heusel <christian@xxxxxxxxx> Acked-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx> Cc: Chengming Zhou <chengming.zhou@xxxxxxxxx> Cc: Dan Streetman <ddstreet@xxxxxxxx> Cc: Richard W.M. Jones <rjones@xxxxxxxxxx> Cc: Seth Jennings <sjenning@xxxxxxxxxx> Cc: Vitaly Wool <vitaly.wool@xxxxxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> [v6.8] Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> (cherry picked from commit 682886ec69d22363819a83ddddd5d66cb5c791e1) Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx> --- 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 diff --git a/mm/zswap.c b/mm/zswap.c index e9c04387195f..69766f2c5a6c 100644 --- a/mm/zswap.c +++ b/mm/zswap.c @@ -653,15 +653,22 @@ static unsigned long zswap_shrinker_count(struct shrinker *shrinker, 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; -- 2.44.0
