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SubjectRe: [v8 PATCH 03/13] mm: vmscan: use shrinker_rwsem to protect shrinker_maps allocation
On Tue, Feb 16, 2021 at 4:13 PM Yang Shi <shy828301@gmail.com> wrote:
>
> Since memcg_shrinker_map_size just can be changed under holding shrinker_rwsem
> exclusively, the read side can be protected by holding read lock, so it sounds
> superfluous to have a dedicated mutex.
>
> Kirill Tkhai suggested use write lock since:
>
> * We want the assignment to shrinker_maps is visible for shrink_slab_memcg().
> * The rcu_dereference_protected() dereferrencing in shrink_slab_memcg(), but
> in case of we use READ lock in alloc_shrinker_maps(), the dereferrencing
> is not actually protected.
> * READ lock makes alloc_shrinker_info() racy against memory allocation fail.
> alloc_shrinker_info()->free_shrinker_info() may free memory right after
> shrink_slab_memcg() dereferenced it. You may say
> shrink_slab_memcg()->mem_cgroup_online() protects us from it? Yes, sure,
> but this is not the thing we want to remember in the future, since this
> spreads modularity.
>
> And a test with heavy paging workload didn't show write lock makes things worse.
>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> Acked-by: Kirill Tkhai <ktkhai@virtuozzo.com>
> Acked-by: Roman Gushchin <guro@fb.com>
> Signed-off-by: Yang Shi <shy828301@gmail.com>

Reviewed-by: Shakeel Butt <shakeelb@google.com>

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