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charset="UTF-8" X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Dec 15, 2020 at 3:07 PM Yang Shi wrote: > > On Mon, Dec 14, 2020 at 7:05 PM Dave Chinner wrote: > > > > On Mon, Dec 14, 2020 at 02:37:20PM -0800, Yang Shi wrote: > > > Now nr_deferred is available on per memcg level for memcg aware shrinkers, so don't need > > > allocate shrinker->nr_deferred for such shrinkers anymore. > > > > > > Signed-off-by: Yang Shi > > > --- > > > mm/vmscan.c | 28 ++++++++++++++-------------- > > > 1 file changed, 14 insertions(+), 14 deletions(-) > > > > > > diff --git a/mm/vmscan.c b/mm/vmscan.c > > > index bce8cf44eca2..8d5bfd818acd 100644 > > > --- a/mm/vmscan.c > > > +++ b/mm/vmscan.c > > > @@ -420,7 +420,15 @@ unsigned long lruvec_lru_size(struct lruvec *lruvec, enum lru_list lru, int zone > > > */ > > > int prealloc_shrinker(struct shrinker *shrinker) > > > { > > > - unsigned int size = sizeof(*shrinker->nr_deferred); > > > + unsigned int size; > > > + > > > + if (is_deferred_memcg_aware(shrinker)) { > > > + if (prealloc_memcg_shrinker(shrinker)) > > > + return -ENOMEM; > > > + return 0; > > > + } > > > + > > > + size = sizeof(*shrinker->nr_deferred); > > > > > > if (shrinker->flags & SHRINKER_NUMA_AWARE) > > > size *= nr_node_ids; > > > @@ -429,26 +437,18 @@ int prealloc_shrinker(struct shrinker *shrinker) > > > if (!shrinker->nr_deferred) > > > return -ENOMEM; > > > > > > - if (shrinker->flags & SHRINKER_MEMCG_AWARE) { > > > - if (prealloc_memcg_shrinker(shrinker)) > > > - goto free_deferred; > > > - } > > > - > > > return 0; > > > - > > > -free_deferred: > > > - kfree(shrinker->nr_deferred); > > > - shrinker->nr_deferred = NULL; > > > - return -ENOMEM; > > > } > > > > I'm trying to put my finger on it, but this seems wrong to me. If > > memcgs are disabled, then prealloc_memcg_shrinker() needs to fail. > > The preallocation code should not care about internal memcg details > > like this. > > > > /* > > * If the shrinker is memcg aware and memcgs are not > > * enabled, clear the MEMCG flag and fall back to non-memcg > > * behaviour for the shrinker. > > */ > > if (shrinker->flags & SHRINKER_MEMCG_AWARE) { > > error = prealloc_memcg_shrinker(shrinker); > > if (!error) > > return 0; > > if (error != -ENOSYS) > > return error; > > > > /* memcgs not enabled! */ > > shrinker->flags &= ~SHRINKER_MEMCG_AWARE; > > } > > > > size = sizeof(*shrinker->nr_deferred); > > .... > > return 0; > > } > > > > This guarantees that only the shrinker instances taht have a > > correctly set up memcg attached to them will have the > > SHRINKER_MEMCG_AWARE flag set. Hence in all the rest of the shrinker > > code, we only ever need to check for SHRINKER_MEMCG_AWARE to > > determine what we should do.... > > Thanks. I see your point. We could move the memcg specific details > into prealloc_memcg_shrinker(). > > It seems we have to acquire shrinker_rwsem before we check and modify > SHIRNKER_MEMCG_AWARE bit if we may clear it. Hi Dave, Is it possible that shrinker register races with shrinker unregister? It seems impossible to me by a quick visual code inspection. But I'm not a VFS expert so I'm not quite sure. If it is impossible the implementation would be quite simple otherwise we need move shrinker_rwsem acquire/release to prealloc_shrinker/free_prealloced_shrinker/unregister_shrinker to protect SHRINKER_MEMCG_AWARE update. > > > > > > void free_prealloced_shrinker(struct shrinker *shrinker) > > > { > > > - if (!shrinker->nr_deferred) > > > + if (is_deferred_memcg_aware(shrinker)) { > > > + unregister_memcg_shrinker(shrinker); > > > return; > > > + } > > > > > > - if (shrinker->flags & SHRINKER_MEMCG_AWARE) > > > - unregister_memcg_shrinker(shrinker); > > > + if (!shrinker->nr_deferred) > > > + return; > > > > > > kfree(shrinker->nr_deferred); > > > shrinker->nr_deferred = NULL; > > > > e.g. then this function can simply do: > > > > { > > if (shrinker->flags & SHRINKER_MEMCG_AWARE) > > return unregister_memcg_shrinker(shrinker); > > kfree(shrinker->nr_deferred); > > shrinker->nr_deferred = NULL; > > } > > > > Cheers, > > > > Dave. > > -- > > Dave Chinner > > david@fromorbit.com