From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Morton Subject: Re: [PATCH v4] mm/memcg: try harder to decrease [memory,memsw].limit_in_bytes Date: Thu, 11 Jan 2018 16:21:34 -0800 Message-ID: <20180111162134.53aa5a44c59689ec0399db57@linux-foundation.org> References: <20180109152622.31ca558acb0cc25a1b14f38c@linux-foundation.org> <20180110124317.28887-1-aryabinin@virtuozzo.com> <20180110143121.cf2a1c5497b31642c9b38b2a@linux-foundation.org> <47856d2b-1534-6198-c2e2-6d2356973bef@virtuozzo.com> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <47856d2b-1534-6198-c2e2-6d2356973bef-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org> Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="us-ascii" To: Andrey Ryabinin Cc: Michal Hocko , Johannes Weiner , Vladimir Davydov , cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Shakeel Butt On Thu, 11 Jan 2018 14:59:23 +0300 Andrey Ryabinin wrote: > On 01/11/2018 01:31 AM, Andrew Morton wrote: > > On Wed, 10 Jan 2018 15:43:17 +0300 Andrey Ryabinin wrote: > > > >> mem_cgroup_resize_[memsw]_limit() tries to free only 32 (SWAP_CLUSTER_MAX) > >> pages on each iteration. This makes practically impossible to decrease > >> limit of memory cgroup. Tasks could easily allocate back 32 pages, > >> so we can't reduce memory usage, and once retry_count reaches zero we return > >> -EBUSY. > >> > >> Easy to reproduce the problem by running the following commands: > >> > >> mkdir /sys/fs/cgroup/memory/test > >> echo $$ >> /sys/fs/cgroup/memory/test/tasks > >> cat big_file > /dev/null & > >> sleep 1 && echo $((100*1024*1024)) > /sys/fs/cgroup/memory/test/memory.limit_in_bytes > >> -bash: echo: write error: Device or resource busy > >> > >> Instead of relying on retry_count, keep retrying the reclaim until > >> the desired limit is reached or fail if the reclaim doesn't make > >> any progress or a signal is pending. > >> > > > > Is there any situation under which that mem_cgroup_resize_limit() can > > get stuck semi-indefinitely in a livelockish state? It isn't very > > obvious that we're protected from this, so perhaps it would help to > > have a comment which describes how loop termination is assured? > > > > We are not protected from this. If tasks in cgroup *indefinitely* generate reclaimable memory at high rate > and user asks to set unreachable limit, like 'echo 4096 > memory.limit_in_bytes', than > try_to_free_mem_cgroup_pages() will return non-zero indefinitely. > > Is that a big deal? At least loop can be interrupted by a signal, and we don't hold any locks here. It may be better to detect this condition, give up and return an error?