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[109.81.94.245]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-49592f39899sm9626204f8f.3.2026.11.09.07.20.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 09 Nov 2026 07:20:31 -0800 (PST) Date: Mon, 9 Nov 2026 16:20:30 +0100 From: Michal Hocko To: Chris Down Cc: Andrew Morton , Johannes Weiner , Roman Gushchin , Shakeel Butt , Muchun Song , Tejun Heo , Sebastian Andrzej Siewior , Zhan Xusheng , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@meta.com Subject: Re: [PATCH v2] mm: memcontrol: Don't lose track of exiting OOM victims Message-ID: References: X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Spam-Status: No, score=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_PASS, SPF_PASS autolearn=ham autolearn_force=no version=4.0.1 X-Rspamd-Server: rspam09 X-Stat-Signature: e6mocahx1dtemrejk7oy8tdrwzb3ifbp X-Rspamd-Queue-Id: 6C0308000E X-Rspam-User: X-HE-Tag: 1794237634-770911 X-HE-Meta: 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 ervadJap 8ZEsD6nF83fOPNAYP1FH1+v4BYXq7tYXdEYB2CeXLe9r3835m6BojmqeHSFOobLqna/ASUDXA6GuxZ8/aLsONmQee2KTmRXdkJYdlge1aMJgLtWRs5i4WOpX+tUVtGbcIk0uv8Ti0/qZSAtQOI6WkJidbXMilzZspiskL34FsjaDLXDKZAjttN5CBqMOtrS26+bSA8HWFsIktVZLRSfsJrsBfOsYrqZUtkxM7Z3eIa4xHCVJ0ru3KAg0uMo6PQcz3MFwlu3n5FqQjOuz1qq5x9pwnztDna4K/5/EuH+V5eLsTD7VCSczrzB+6eJEzog1yNlFSPTMsMLugupdYY4lC1yx2yguf2You/eBSA/ZKwnWneUIgs1o6symvZx66TCE3nFltAHZ23/BXMu9tqA+/VVJiYG8jdiAbm8a8nOd6qN+Bobn9Bs4ILpdMm39DF7dqTVCo Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On Sun 08-11-26 00:41:37, Chris Down wrote: > When oom_evaluate_task() comes across a victim we've already killed, it > backs off until the victim's mm has MMF_OOM_SKIP. This gives the victim > a chance to free some memory before we kill anything else. However, > since commit a72f73c4dd9b ("cgroup: Don't expose dead tasks in cgroup"), > css_task_iter skips exiting tasks once signal->live reaches zero. That > happens before exit_mm(), so a single-threaded victim can disappear > from the memcg scan while it still has all its memory. > > This can kill another process when the first kill would have been > enough to recover, and we've seen it happen in production. If the > second victim is in a cgroup with memory.oom.group enabled, the extra > kill can take down the whole workload. > > In tests I used a 64 MiB memcg with swap disabled. I started a process > that faults in 44 MiB of anonymous memory, submits a 4 KiB direct AIO > read to null_blk, and goes to sleep. I set null_blk to take 10 seconds > to complete the read so the victim would have to wait for it in > exit_aio(). Two seconds after starting the first process, I started a > second one that tries to fault in 40 MiB. > > The 40 MiB allocation hits memory.max partway through and triggers an > OOM kill. The OOM killer picks the first process, which has more > resident memory. With the read still pending, the victim blocks in > exit_aio() before it can tear down its address space. > > memory.events showed oom_kill 2 in three runs. In one of them the first > process hadn't submitted the read yet when it was killed. The slow read > just makes the window easier to hit. > > The cgroup iterator hides these tasks so that cgroup.procs doesn't > show processes that waitpid() may already have reaped. That's fine for > cgroup.procs, but the OOM scan needs to see a victim even after its last > thread has started exiting. We already have CSS_TASK_ITER_WITH_DEAD > for sched_ext, so use it for mem_cgroup_scan_tasks() as well. > > With this applied, OOM selection backs off while the first victim still > has memory to free. The 40 MiB allocation completes and memory.events > shows oom_kill 1 in three test runs. > > Fixes: a72f73c4dd9b ("cgroup: Don't expose dead tasks in cgroup") > Cc: > Signed-off-by: Chris Down Acked-by: Michal Hocko Thanks! > --- > v2: Give more concrete details in changelog (akpm) > > mm/memcontrol.c | 8 +++++++- > 1 file changed, 7 insertions(+), 1 deletion(-) > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > index 84f4a8fd4d16..cb7cfae9ce5a 100644 > --- a/mm/memcontrol.c > +++ b/mm/memcontrol.c > @@ -1451,7 +1451,13 @@ void mem_cgroup_scan_tasks(struct mem_cgroup *memcg, > struct css_task_iter it; > struct task_struct *task; > > - css_task_iter_start(&iter->css, CSS_TASK_ITER_PROCS, &it); > + /* > + * An OOM victim can start exiting well before its mm is > + * torn down. Keep it visible so oom_evaluate_task() can > + * back off while there's still memory to free. > + */ > + css_task_iter_start(&iter->css, CSS_TASK_ITER_PROCS | > + CSS_TASK_ITER_WITH_DEAD, &it); > while (!ret && (task = css_task_iter_next(&it))) { > ret = fn(task, arg); > /* Avoid potential softlockup warning */ > > base-commit: fa4ad798f6f681031775266044ba1b0cf3c670bd -- Michal Hocko SUSE Labs