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/mm-zswap-fix-shrinker-null-crash-with-cgroup_disable=memory.patch
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@xxxxxxxxxxx>
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@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>
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: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> [v6.8]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
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@xxxxxxxxxxx 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
