| Message ID | 20210203172042.800474-4-shy828301@gmail.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Make shrinker's nr_deferred memcg aware | expand |
On 03.02.2021 20:20, 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 <vbabka@suse.cz> > Signed-off-by: Yang Shi <shy828301@gmail.com> Acked-by: Kirill Tkhai <ktkhai@virtuozzo.com> > --- > 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); > - > 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); > if (!root_mem_cgroup) > - goto unlock; > + goto out; > > memcg = mem_cgroup_iter(NULL, NULL, NULL); > do { > @@ -286,13 +282,13 @@ static int expand_shrinker_maps(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; > } > >
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); - 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); if (!root_mem_cgroup) - goto unlock; + goto out; memcg = mem_cgroup_iter(NULL, NULL, NULL); do { @@ -286,13 +282,13 @@ static int expand_shrinker_maps(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; }
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> Signed-off-by: Yang Shi <shy828301@gmail.com> --- mm/vmscan.c | 16 ++++++---------- 1 file changed, 6 insertions(+), 10 deletions(-)