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charset=us-ascii Content-Disposition: inline In-Reply-To: <6ffd6aa1-2c55-f4d3-a60a-56786d40531a@virtuozzo.com> 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 Thu, Dec 10, 2020 at 06:17:54PM +0300, Kirill Tkhai wrote: > On 10.12.2020 18:13, Johannes Weiner wrote: > > On Wed, Dec 09, 2020 at 09:32:37AM -0800, Yang Shi wrote: > >> On Wed, Dec 9, 2020 at 7:42 AM Kirill Tkhai wrote: > >>> > >>> On 08.12.2020 20:13, Yang Shi wrote: > >>>> On Thu, Dec 3, 2020 at 3:40 AM Kirill Tkhai wrote: > >>>>> > >>>>> On 02.12.2020 21:27, Yang Shi wrote: > >>>>>> Use per memcg's nr_deferred for memcg aware shrinkers. The shrinker's nr_deferred > >>>>>> will be used in the following cases: > >>>>>> 1. Non memcg aware shrinkers > >>>>>> 2. !CONFIG_MEMCG > >>>>>> 3. memcg is disabled by boot parameter > >>>>>> > >>>>>> Signed-off-by: Yang Shi > >>>>>> --- > >>>>>> mm/vmscan.c | 88 +++++++++++++++++++++++++++++++++++++++++++++++++---- > >>>>>> 1 file changed, 82 insertions(+), 6 deletions(-) > >>>>>> > >>>>>> diff --git a/mm/vmscan.c b/mm/vmscan.c > >>>>>> index cba0bc8d4661..d569fdcaba79 100644 > >>>>>> --- a/mm/vmscan.c > >>>>>> +++ b/mm/vmscan.c > >>>>>> @@ -203,6 +203,12 @@ static DECLARE_RWSEM(shrinker_rwsem); > >>>>>> static DEFINE_IDR(shrinker_idr); > >>>>>> static int shrinker_nr_max; > >>>>>> > >>>>>> +static inline bool is_deferred_memcg_aware(struct shrinker *shrinker) > >>>>>> +{ > >>>>>> + return (shrinker->flags & SHRINKER_MEMCG_AWARE) && > >>>>>> + !mem_cgroup_disabled(); > >>>>>> +} > >>>>>> + > >>>>>> static int prealloc_memcg_shrinker(struct shrinker *shrinker) > >>>>>> { > >>>>>> int id, ret = -ENOMEM; > >>>>>> @@ -271,7 +277,58 @@ static bool writeback_throttling_sane(struct scan_control *sc) > >>>>>> #endif > >>>>>> return false; > >>>>>> } > >>>>>> + > >>>>>> +static inline long count_nr_deferred(struct shrinker *shrinker, > >>>>>> + struct shrink_control *sc) > >>>>>> +{ > >>>>>> + bool per_memcg_deferred = is_deferred_memcg_aware(shrinker) && sc->memcg; > >>>>>> + struct memcg_shrinker_deferred *deferred; > >>>>>> + struct mem_cgroup *memcg = sc->memcg; > >>>>>> + int nid = sc->nid; > >>>>>> + int id = shrinker->id; > >>>>>> + long nr; > >>>>>> + > >>>>>> + if (!(shrinker->flags & SHRINKER_NUMA_AWARE)) > >>>>>> + nid = 0; > >>>>>> + > >>>>>> + if (per_memcg_deferred) { > >>>>>> + deferred = rcu_dereference_protected(memcg->nodeinfo[nid]->shrinker_deferred, > >>>>>> + true); > >>>>> > >>>>> My comment is about both 5/9 and 6/9 patches. > >>>> > >>>> Sorry for the late reply, I don't know why Gmail filtered this out to spam. > >>>> > >>>>> > >>>>> shrink_slab_memcg() races with mem_cgroup_css_online(). A visibility of CSS_ONLINE flag > >>>>> in shrink_slab_memcg()->mem_cgroup_online() does not guarantee that you will see > >>>>> memcg->nodeinfo[nid]->shrinker_deferred != NULL in count_nr_deferred(). This may occur > >>>>> because of processor reordering on !x86 (there is no a common lock or memory barriers). > >>>>> > >>>>> Regarding to shrinker_map this is not a problem due to map check in shrink_slab_memcg(). > >>>>> The map can't be NULL there. > >>>>> > >>>>> Regarding to shrinker_deferred you should prove either this is not a problem too, > >>>>> or to add proper synchronization (maybe, based on barriers) or to add some similar check > >>>>> (maybe, in shrink_slab_memcg() too). > >>>> > >>>> It seems shrink_slab_memcg() might see shrinker_deferred as NULL > >>>> either due to the same reason. I don't think there is a guarantee it > >>>> won't happen. > >>>> > >>>> We just need guarantee CSS_ONLINE is seen after shrinker_maps and > >>>> shrinker_deferred are allocated, so I'm supposed barriers before > >>>> "css->flags |= CSS_ONLINE" should work. > >>>> > >>>> So the below patch may be ok: > >>>> > >>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c > >>>> index df128cab900f..9f7fb0450d69 100644 > >>>> --- a/mm/memcontrol.c > >>>> +++ b/mm/memcontrol.c > >>>> @@ -5539,6 +5539,12 @@ static int mem_cgroup_css_online(struct > >>>> cgroup_subsys_state *css) > >>>> return -ENOMEM; > >>>> } > >>>> > >>>> > >>>> + /* > >>>> + * Barrier for CSS_ONLINE, so that shrink_slab_memcg() sees > >>>> shirnker_maps > >>>> + * and shrinker_deferred before CSS_ONLINE. > >>>> + */ > >>>> + smp_mb(); > >>>> + > >>>> /* Online state pins memcg ID, memcg ID pins CSS */ > >>>> refcount_set(&memcg->id.ref, 1); > >>>> css_get(css); > >>> > >>> smp barriers synchronize data access from different cpus. They should go in a pair. > >>> In case of you add the smp barrier into mem_cgroup_css_online(), we should also > >>> add one more smp barrier in another place, which we want to synchonize with this. > >>> Also, every place should contain a comment referring to its pair: "Pairs with...". > >> > >> Thanks, I think you are correct. Looked into it further, it seems the > >> race pattern looks like: > >> > >> CPU A CPU B > >> store shrinker_maps pointer load CSS_ONLINE > >> store CSS_ONLINE load shrinker_maps pointer > >> > >> By checking the memory-barriers document, it seems we need write > >> barrier/read barrier pair as below: > >> > >> CPU A CPU B > >> store shrinker_maps pointer load CSS_ONLINE > >> > >> store CSS_ONLINE load shrinker_maps pointer > >> > >> > >> So, the patch should look like: > >> > >> diff --git a/mm/memcontrol.c b/mm/memcontrol.c > >> index df128cab900f..489c0a84f82b 100644 > >> --- a/mm/memcontrol.c > >> +++ b/mm/memcontrol.c > >> @@ -5539,6 +5539,13 @@ static int mem_cgroup_css_online(struct > >> cgroup_subsys_state *css) > >> return -ENOMEM; > >> } > >> > >> + /* > >> + * Barrier for CSS_ONLINE, so that shrink_slab_memcg() sees > >> shirnker_maps > >> + * and shrinker_deferred before CSS_ONLINE. It pairs with the > >> read barrier > >> + * in shrink_slab_memcg(). > >> + */ > >> + smp_wmb(); > > > > Is there a reason why the shrinker allocations aren't done in > > .css_alloc()? That would take care of all necessary ordering: > > The reason is that allocations have to be made in a place, where > mem-cgroup_iter() can't miss it, since memcg_expand_shrinker_maps() > shouldn't miss allocated shrinker maps. I see, because we could have this: .css_alloc() memcg_alloc_shrinker_maps() down_read(&shrinker_sem) map = alloc(shrinker_nr_max * sizeof(long)); rcu_assign_pointer(memcg->...->shrinker_map = map); up_read(&shrinker_sem); register_shrinker() down_write(&shrinker_sem) shrinker_nr_max = id + 1; memcg_expand_shrinker_maps() for_each_mem_cgroup() realloc up_write(&shrinker_sem) list_add_tail_rcu(&css->sibling, &parent->children); /* boom: missed new shrinker, map too small */ Thanks for the clarification.