Re: [PATCH] memcg: protect concurrent access to mem_cgroup_idr

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, Aug 02, 2024 at 04:58:22PM -0700, Shakeel Butt wrote:
> The commit 73f576c04b94 ("mm: memcontrol: fix cgroup creation failure
> after many small jobs") decoupled the memcg IDs from the CSS ID space to
> fix the cgroup creation failures. It introduced IDR to maintain the
> memcg ID space. The IDR depends on external synchronization mechanisms
> for modifications. For the mem_cgroup_idr, the idr_alloc() and
> idr_replace() happen within css callback and thus are protected through
> cgroup_mutex from concurrent modifications. However idr_remove() for
> mem_cgroup_idr was not protected against concurrency and can be run
> concurrently for different memcgs when they hit their refcnt to zero.
> Fix that.
> 
> We have been seeing list_lru based kernel crashes at a low frequency in
> our fleet for a long time. These crashes were in different part of
> list_lru code including list_lru_add(), list_lru_del() and reparenting
> code. Upon further inspection, it looked like for a given object (dentry
> and inode), the super_block's list_lru didn't have list_lru_one for the
> memcg of that object. The initial suspicions were either the object is
> not allocated through kmem_cache_alloc_lru() or somehow
> memcg_list_lru_alloc() failed to allocate list_lru_one() for a memcg but
> returned success. No evidence were found for these cases.
> 
> Looking more deeper, we started seeing situations where valid memcg's id
> is not present in mem_cgroup_idr and in some cases multiple valid memcgs
> have same id and mem_cgroup_idr is pointing to one of them. So, the most
> reasonable explanation is that these situations can happen due to race
> between multiple idr_remove() calls or race between
> idr_alloc()/idr_replace() and idr_remove(). These races are causing
> multiple memcgs to acquire the same ID and then offlining of one of them
> would cleanup list_lrus on the system for all of them. Later access from
> other memcgs to the list_lru cause crashes due to missing list_lru_one.
> 
> Fixes: 73f576c04b94 ("mm: memcontrol: fix cgroup creation failure after many small jobs")
> Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>

Great catch. This has been busted for ages, but the race is so
unlikely that it stayed low profile.

Acked-by: Johannes Weiner <hannes@cmpxchg.org>

It probably should be Cc: stable as well.