The patch titled Subject: mm: memcontrol: fix blocking rstat function called from atomic cgroup1 thresholding code has been added to the -mm tree. Its filename is mm-memcontrol-fix-blocking-rstat-function-called-from-atomic-cgroup1-thresholding-code.patch

This patch should soon appear at https://ozlabs.org/~akpm/mmots/broken-out/mm-memcontrol-fix-blocking-rstat-f... and later at https://ozlabs.org/~akpm/mmotm/broken-out/mm-memcontrol-fix-blocking-rstat-f...

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------------------------------------------------------ From: Johannes Weiner hannes@cmpxchg.org Subject: mm: memcontrol: fix blocking rstat function called from atomic cgroup1 thresholding code

Dan Carpenter reports:

The patch 2d146aa3aa84: "mm: memcontrol: switch to rstat" from Apr 29, 2021, leads to the following static checker warning:

kernel/cgroup/rstat.c:200 cgroup_rstat_flush() warn: sleeping in atomic context

mm/memcontrol.c 3572 static unsigned long mem_cgroup_usage(struct mem_cgroup *memcg, bool swap) 3573 { 3574 unsigned long val; 3575 3576 if (mem_cgroup_is_root(memcg)) { 3577 cgroup_rstat_flush(memcg->css.cgroup); ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

This is from static analysis and potentially a false positive. The problem is that mem_cgroup_usage() is called from __mem_cgroup_threshold() which holds an rcu_read_lock(). And the cgroup_rstat_flush() function can sleep.

3578 val = memcg_page_state(memcg, NR_FILE_PAGES) + 3579 memcg_page_state(memcg, NR_ANON_MAPPED); 3580 if (swap) 3581 val += memcg_page_state(memcg, MEMCG_SWAP); 3582 } else { 3583 if (!swap) 3584 val = page_counter_read(&memcg->memory); 3585 else 3586 val = page_counter_read(&memcg->memsw); 3587 } 3588 return val; 3589 }

__mem_cgroup_threshold() indeed holds the rcu lock. In addition, the thresholding code is invoked during stat changes, and those contexts have irqs disabled as well. If the lock breaking occurs inside the flush function, it will result in a sleep from an atomic context.

Use the irqsafe flushing variant in mem_cgroup_usage() to fix this.

Link: https://lkml.kernel.org/r/20210726150019.251820-1-hannes@cmpxchg.org Fixes: 2d146aa3aa84 ("mm: memcontrol: switch to rstat") Signed-off-by: Johannes Weiner hannes@cmpxchg.org Reported-by: Dan Carpenter dan.carpenter@oracle.com Acked-by: Chris Down chris@chrisdown.name Reviewed-by: Rik van Riel riel@surriel.com Acked-by: Michal Hocko mhocko@suse.com Reviewed-by: Shakeel Butt shakeelb@google.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/memcontrol.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/mm/memcontrol.c~mm-memcontrol-fix-blocking-rstat-function-called-from-atomic-cgroup1-thresholding-code +++ a/mm/memcontrol.c @@ -3574,7 +3574,8 @@ static unsigned long mem_cgroup_usage(st unsigned long val;

if (mem_cgroup_is_root(memcg)) { - cgroup_rstat_flush(memcg->css.cgroup); + /* mem_cgroup_threshold() calls here from irqsafe context */ + cgroup_rstat_flush_irqsafe(memcg->css.cgroup); val = memcg_page_state(memcg, NR_FILE_PAGES) + memcg_page_state(memcg, NR_ANON_MAPPED); if (swap) _

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

mm-memcontrol-fix-blocking-rstat-function-called-from-atomic-cgroup1-thresholding-code.patch mm-remove-irqsave-restore-locking-from-contexts-with-irqs-enabled.patch fs-drop_caches-fix-skipping-over-shadow-cache-inodes.patch fs-inode-count-invalidated-shadow-pages-in-pginodesteal.patch vfs-keep-inodes-with-page-cache-off-the-inode-shrinker-lru.patch