On Tue, Feb 16, 2021 at 4:13 PM Yang Shi <shy828301@xxxxxxxxx> 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@xxxxxxx> > Acked-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> > Acked-by: Roman Gushchin <guro@xxxxxx> > Signed-off-by: Yang Shi <shy828301@xxxxxxxxx> Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
