From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrey Ryabinin Subject: Re: [PATCH v4] mm/memcg: try harder to decrease [memory,memsw].limit_in_bytes Date: Fri, 12 Jan 2018 12:08:12 +0300 Message-ID: <8f706bc5-cc9c-01f5-1918-41cd0501f4f0@virtuozzo.com> References: <20180109152622.31ca558acb0cc25a1b14f38c@linux-foundation.org> <20180110124317.28887-1-aryabinin@virtuozzo.com> <20180110143121.cf2a1c5497b31642c9b38b2a@linux-foundation.org> <47856d2b-1534-6198-c2e2-6d2356973bef@virtuozzo.com> <20180111162134.53aa5a44c59689ec0399db57@linux-foundation.org> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=virtuozzo.com; s=selector1; h=From:Date:Subject:Message-ID:Content-Type:MIME-Version; bh=J/EVSyLmUhrzuB9g5gH3Vl6gxKd4rYpTfKWKWiJtsb4=; b=ZPkk7jt+jzaipnBPD6u9XwCmbI5vbiAhBim7yCtMgvu7rejvJfDk8zJonS5zOR/rWje/vbmdFF56hv5ac00sSJXr4s4HGvmNGcb/qsxCwf4E9Vljw124g/eQ12qRHn/yeAT+CzLKvmqkeYKrJKRvfpxVjTtYszJxBEodyaaAxxQ= In-Reply-To: <20180111162134.53aa5a44c59689ec0399db57@linux-foundation.org> Content-Language: en-US Sender: owner-linux-mm@kvack.org List-ID: Content-Type: text/plain; charset="us-ascii" To: Andrew Morton Cc: Michal Hocko , Johannes Weiner , Vladimir Davydov , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Shakeel Butt On 01/12/2018 03:21 AM, Andrew Morton wrote: > 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? > That's basically what how v1 worked, "if (curusage >= oldusage)" used to be the way to detect this potential livelock. So we can just go back to it? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org