Messages in this thread | | | Subject | Re: [v5 PATCH 03/11] mm: vmscan: use shrinker_rwsem to protect shrinker_maps allocation | From | Vlastimil Babka <> | Date | Thu, 28 Jan 2021 17:19:08 +0100 |
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On 1/28/21 12:33 AM, Yang Shi 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. > > Signed-off-by: Yang Shi <shy828301@gmail.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
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