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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-Mailing-List: cgroups@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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