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[PATCH] memcg: allow exiting tasks to write back data to swap Content-Language: en-GB To: Rik van Riel , Johannes Weiner Cc: Michal Hocko , Roman Gushchin , Shakeel Butt , Muchun Song , Andrew Morton , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@meta.com, Nhat Pham , Yosry Ahmed References: <20241211105336.380cb545@fangorn> <766a28a1-c82b-46fd-b3b0-fe3b6024f462@nvidia.com> <1764e2fc8cff5b07aa9df1ae90a13986a3949250.camel@surriel.com> From: Balbir Singh In-Reply-To: <1764e2fc8cff5b07aa9df1ae90a13986a3949250.camel@surriel.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-ClientProxiedBy: SA0PR11CA0094.namprd11.prod.outlook.com (2603:10b6:806:d1::9) To PH8PR12MB7277.namprd12.prod.outlook.com (2603:10b6:510:223::13) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH8PR12MB7277:EE_|PH7PR12MB5927:EE_ X-MS-Office365-Filtering-Correlation-Id: dce8184e-2293-4e6d-4ce4-08dd1a5caac8 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U/SAtZNLB+mNAE1rt3G1WxY7An0nqi/vcG5DGmUyXuaU1oqsm8xIjgVhaXIhpznnEyQ3MtnJsW5VsPXa8WEmTZjEtXiBJiucTY1xfoQy030KuK9Hz3pmdGoQFYhd4w9FYLyyrzMz8uv9rhwsB5CsIy3k14JBWJf3m07vnUgRiHE+yBxMAgIpr0u+1Mf6qu9RFAg9fS9h16M4p4oBDVuDt3E6cbYXtBB68aqwoz5uKvoe0laj7kePiyU30+j0cJby4dvk/8/3Phb9RKvENjscTz9Tyxf08KiposJSM3iXJ0sQD6mjmq4BIvyL+DU7O6izFt35rIoVujiz0WPyqlJ4MhOeYce+SgsVCwrxJEZgAsPYYbSjalxvV/mJO4w== 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: List-Subscribe: List-Unsubscribe: On 12/12/24 12:21, Rik van Riel wrote: > On Thu, 2024-12-12 at 10:15 +1100, Balbir Singh wrote: >> On 12/12/24 02:53, Rik van Riel wrote: >>> >>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c >>> index 7b3503d12aaf..03d77e93087e 100644 >>> --- a/mm/memcontrol.c >>> +++ b/mm/memcontrol.c >>> @@ -5371,6 +5371,15 @@ bool >>> mem_cgroup_zswap_writeback_enabled(struct mem_cgroup *memcg) >>> if (!zswap_is_enabled()) >>> return true; >>> >>> + /* >>> + * Always allow exiting tasks to push data to swap. A >>> process in >>> + * the middle of exit cannot get OOM killed, but may need >>> to push >>> + * uncompressible data to swap in order to get the cgroup >>> memory >>> + * use below the limit, and make progress with the exit. >>> + */ >>> + if ((current->flags & PF_EXITING) && memcg == >>> mem_cgroup_from_task(current)) >>> + return true; >>> + >>> for (; memcg; memcg = parent_mem_cgroup(memcg)) >>> if (!READ_ONCE(memcg->zswap_writeback)) >>> return false; >> >> Rik, >> >> I am unable to understand the motivation here, so we want >> mem_cgroup_zswap_writeback_enabled() to return true, it only >> returns false if a memcg in the hierarchy has zswap_writeback >> set to 0 (false). In my git-grep I can't seem to find how/why >> that may be the case. I can see memcg starts of with the value >> set to true, if CONFIG_ZSWAP is enabled. >> >> Your changelog above makes sense, but I am unable to map it to >> the code changes. >> > > Wait, are you asking about the code that I'm > adding, or about the code that was already > there? > > I want to add the code that allows zswap > writeback if the reclaiming task is exiting, > and in the same cgroup as the to be written > back memory. > I was asking about this change (this patch), I know that the return true will help avoid the PAGE_ACTIVATE path, but I am not sure why this function will return false if CONFIG_ZSWAP is enabled (unless zswap_writeback is turned off in one of the groups) Balbir