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Tue, 09 Feb 2021 15:28:41 -0800 (PST) MIME-Version: 1.0 References: <20210209174646.1310591-1-shy828301@gmail.com> <20210209174646.1310591-4-shy828301@gmail.com> <20210209203307.GF524633@carbon.DHCP.thefacebook.com> In-Reply-To: <20210209203307.GF524633@carbon.DHCP.thefacebook.com> From: Yang Shi Date: Tue, 9 Feb 2021 15:28:30 -0800 Message-ID: Subject: Re: [v7 PATCH 03/12] mm: vmscan: use shrinker_rwsem to protect shrinker_maps allocation To: Roman Gushchin Cc: Kirill Tkhai , Vlastimil Babka , Shakeel Butt , Dave Chinner , Johannes Weiner , Michal Hocko , Andrew Morton , Linux MM , Linux FS-devel Mailing List , Linux Kernel Mailing List Content-Type: text/plain; charset="UTF-8" X-Stat-Signature: f73gem4oxzof7ow1nkbj14yrmbxwh1tw X-Rspamd-Server: rspam01 X-Rspamd-Queue-Id: 582E880191DF Received-SPF: none (gmail.com>: No applicable sender policy available) receiver=imf08; identity=mailfrom; envelope-from=""; helo=mail-ej1-f54.google.com; client-ip=209.85.218.54 X-HE-DKIM-Result: pass/pass X-HE-Tag: 1612913321-685154 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, Feb 9, 2021 at 12:33 PM Roman Gushchin wrote: > > On Tue, Feb 09, 2021 at 09:46:37AM -0800, 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. > > > > Acked-by: Vlastimil Babka > > Acked-by: Kirill Tkhai > > Signed-off-by: Yang Shi > > Acked-by: Roman Gushchin > > with a small nit (below): > > > --- > > mm/vmscan.c | 16 ++++++---------- > > 1 file changed, 6 insertions(+), 10 deletions(-) > > > > diff --git a/mm/vmscan.c b/mm/vmscan.c > > index 96b08c79f18d..e4ddaaaeffe2 100644 > > --- a/mm/vmscan.c > > +++ b/mm/vmscan.c > > @@ -187,7 +187,6 @@ static DECLARE_RWSEM(shrinker_rwsem); > > #ifdef CONFIG_MEMCG > > > > static int memcg_shrinker_map_size; > > -static DEFINE_MUTEX(memcg_shrinker_map_mutex); > > > > static void free_shrinker_map_rcu(struct rcu_head *head) > > { > > @@ -200,8 +199,6 @@ static int expand_one_shrinker_map(struct mem_cgroup *memcg, > > struct memcg_shrinker_map *new, *old; > > int nid; > > > > - lockdep_assert_held(&memcg_shrinker_map_mutex); > > - > > Why not check that shrinker_rwsem is down here? No special reason, just because we know it was acquired before. We could add the check, but not here. I think it'd be better to have the assert in expand_shrinker_maps() since the rwsem was acquired before calling it. > > > for_each_node(nid) { > > old = rcu_dereference_protected( > > mem_cgroup_nodeinfo(memcg, nid)->shrinker_map, true); > > @@ -249,7 +246,7 @@ int alloc_shrinker_maps(struct mem_cgroup *memcg) > > if (mem_cgroup_is_root(memcg)) > > return 0; > > > > - mutex_lock(&memcg_shrinker_map_mutex); > > + down_write(&shrinker_rwsem); > > size = memcg_shrinker_map_size; > > for_each_node(nid) { > > map = kvzalloc_node(sizeof(*map) + size, GFP_KERNEL, nid); > > @@ -260,7 +257,7 @@ int alloc_shrinker_maps(struct mem_cgroup *memcg) > > } > > rcu_assign_pointer(memcg->nodeinfo[nid]->shrinker_map, map); > > } > > - mutex_unlock(&memcg_shrinker_map_mutex); > > + up_write(&shrinker_rwsem); > > > > return ret; > > } > > @@ -275,9 +272,8 @@ static int expand_shrinker_maps(int new_id) > > if (size <= old_size) > > return 0; > > > > - mutex_lock(&memcg_shrinker_map_mutex); > > And here as well. It will make the locking model more obvious and will help > to avoid errors in the future. > > > if (!root_mem_cgroup) > > - goto unlock; > > + goto out; > > > > memcg = mem_cgroup_iter(NULL, NULL, NULL); > > do { > > @@ -286,13 +282,13 @@ static int int new_id) > > ret = expand_one_shrinker_map(memcg, size, old_size); > > if (ret) { > > mem_cgroup_iter_break(NULL, memcg); > > - goto unlock; > > + goto out; > > } > > } while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL); > > -unlock: > > +out: > > if (!ret) > > memcg_shrinker_map_size = size; > > - mutex_unlock(&memcg_shrinker_map_mutex); > > + > > return ret; > > } > > > > -- > > 2.26.2 > >