On Wed, Apr 17, 2024 at 07:18:14PM +0200, Christian Heusel wrote: > On 24/04/17 10:33AM, Johannes Weiner wrote: > > > > Christian, can you please test the below patch on top of current > > upstream? > > > > Hey Johannes, > > I have applied your patch on top of 6.9-rc4 and it did solve the crash for > me, thanks for hacking together a fix so quickly! 🤗 > > Tested-By: Christian Heusel <christian@xxxxxxxxx> Thanks for confirming it, and sorry about the breakage. Andrew, can you please use the updated changelog below? --- >From 52f67f5fab6a743c2aedfc8e04a582a9d1025c28 Mon Sep 17 00:00:00 2001 From: Johannes Weiner <hannes@xxxxxxxxxxx> Date: Thu, 18 Apr 2024 08:26:28 -0400 Subject: [PATCH] mm: zswap: fix shrinker NULL crash with cgroup_disable=memory 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 Fixes: b5ba474f3f51 ("zswap: shrink zswap pool based on memory pressure") Cc: stable@xxxxxxxxxxxxxxx [v6.8] Link: https://lkml.kernel.org/r/20240417143324.GA1055428@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> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx> --- mm/zswap.c | 25 ++++++++++++++++--------- 1 file changed, 16 insertions(+), 9 deletions(-) diff --git a/mm/zswap.c b/mm/zswap.c index caed028945b0..6f8850c44b61 100644 --- a/mm/zswap.c +++ b/mm/zswap.c @@ -1331,15 +1331,22 @@ 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 = zswap_pool_total_size >> PAGE_SHIFT; - nr_stored = atomic_read(&zswap_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 = zswap_pool_total_size >> PAGE_SHIFT; + nr_stored = atomic_read(&zswap_nr_stored); + } if (!nr_stored) return 0; -- 2.44.0
