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[109.81.32.81]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-48e71b9a3b1sm20935867f8f.44.2026.11.02.02.12.45 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 02 Nov 2026 02:12:45 -0800 (PST) Date: Mon, 2 Nov 2026 11:12:42 +0100 From: Michal Hocko To: Zhan Xusheng Cc: akpm@linux-foundation.org, hannes@cmpxchg.org, roman.gushchin@linux.dev, shakeel.butt@linux.dev, muchun.song@linux.dev, penguin-kernel@i-love.sakura.ne.jp, tj@kernel.org, mkoutny@suse.com, cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, zhanxusheng@xiaomi.com, stable@vger.kernel.org Subject: Re: [PATCH] mm: memcontrol: don't miss processes whose leader has exited in OOM scans Message-ID: References: <20261026120248.2188024-1-zhanxusheng1024@gmail.com> X-Mailing-List: stable@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: <20261026120248.2188024-1-zhanxusheng1024@gmail.com> On Mon 26-10-26 20:02:48, Zhan Xusheng wrote: > From: Zhan Xusheng > > mem_cgroup_scan_tasks() walks with CSS_TASK_ITER_PROCS, so it sees only > thread group leaders, plus a dying leader that still has live threads. > Moving a process whose leader has exited, by writing its PID to > cgroup.procs, moves the live threads but leaves the leader behind, as > cgroup_migrate_add_task() skips exiting tasks. Is this a desirable/intended behavior? Shouldn't we move the dead leader along with the rest? > The destination's OOM killer then cannot see the process. If it is > alone there, every attempt ends with "Out of memory and no killable > processes..." and the charge is retried indefinitely, and > memory.oom.group kills nothing; otherwise another process is killed in > its place. The source's OOM killer still finds the leader, reaches the > moved threads' mm through find_lock_task_mm(), and kills a process that > has left the memcg. While you patch works around this seems like something to address in the cgroup core. Should CSS_TASK_ITER_PROCS iteration see all processes regardless leader being alive after or prior the migration? AFAIU the thread group belongs to the new cgroup in both cases. > > @@ -1452,9 +1482,10 @@ 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); > + css_task_iter_start(&iter->css, 0, &it); Shouldn't this be CSS_TASK_ITER_THREADED instead of 0? > while (!ret && (task = css_task_iter_next(&it))) { > - ret = fn(task, arg); > + if (!mem_cgroup_scan_skip(task)) > + ret = fn(task, arg); > /* Avoid potential softlockup warning */ > cond_resched(); > } > > base-commit: 6b4b543fa18757297ac756a6fe6e64f506b7844a > -- > 2.43.0 -- Michal Hocko SUSE Labs