The patch titled Subject: mm: zswap: fix shrinker NULL crash with cgroup_disable=memory has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-zswap-fix-shrinker-null-crash-with-cgroup_disable=memory.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-hotfixes-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: Johannes Weiner hannes@cmpxchg.org Subject: mm: zswap: fix shrinker NULL crash with cgroup_disable=memory Date: Thu, 18 Apr 2024 08:26:28 -0400

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 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: Yosry Ahmed yosryahmed@google.com Cc: stable@vger.kernel.org [v6.8] Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/zswap.c | 25 ++++++++++++++++--------- 1 file changed, 16 insertions(+), 9 deletions(-)

--- a/mm/zswap.c~mm-zswap-fix-shrinker-null-crash-with-cgroup_disable=memory +++ a/mm/zswap.c @@ -1331,15 +1331,22 @@ 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 = 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; _

Patches currently in -mm which might be from hannes@cmpxchg.org are

mm-zswap-fix-shrinker-null-crash-with-cgroup_disable=memory.patch mm-zswap-optimize-zswap-pool-size-tracking.patch mm-zpool-return-pool-size-in-pages.patch mm-page_alloc-remove-pcppage-migratetype-caching.patch mm-page_alloc-optimize-free_unref_folios.patch mm-page_alloc-fix-up-block-types-when-merging-compatible-blocks.patch mm-page_alloc-move-free-pages-when-converting-block-during-isolation.patch mm-page_alloc-fix-move_freepages_block-range-error.patch mm-page_alloc-fix-freelist-movement-during-block-conversion.patch mm-page_alloc-close-migratetype-race-between-freeing-and-stealing.patch mm-page_isolation-prepare-for-hygienic-freelists.patch mm-page_isolation-prepare-for-hygienic-freelists-fix.patch mm-page_alloc-consolidate-free-page-accounting.patch mm-page_alloc-consolidate-free-page-accounting-fix.patch mm-page_alloc-consolidate-free-page-accounting-fix-2.patch mm-page_alloc-batch-vmstat-updates-in-expand.patch