From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=91.218.175.206 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793068942; b=1isa6+TBcF5bfKCUcYAz96x6NS91Ux1ALnm7bWuwp3JfeUXxgesiOKfwwsFhRzD9D//O QHEFHXvIV0D1gkR5iUIdllmIND+NDmB+2WB9v5I/v1rZnlV65ftaaCNgYoyOh5VeH0dMG mtHP6mlMSv010ZAdzBGZwA9c6kP0Q4tyUv/o5V5QxueT1QHQKai+nw2nzUhcwF7L/QEz4 gm5F7HvUGKo6AFxP67dPbooBLQ+LVe4CP5yuMFgzKeHQAJZ6GTnxMGD0I82HyH3q0/yAz jUwwa4DiBIoHC/pRIqpma2fUAVScJuxyAWefH/q7wMEAU5MLZUvjoo1LQtgM/1NK6Fw== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793068942; h=DMARC-Filter:X-Envelope-To:DKIM-Signature:X-Envelope-To:Received: X-Mizu-Trace-ID:X-Migadu-Flow:Content-Type:Mime-Version:Subject:From: In-Reply-To:Date:Cc:Content-Transfer-Encoding:Message-Id:References: To:X-Mailer; bh=dxplr2PW9T2cb7PCCoveVu/o9kyEMHS33OhsZEP7yEg=; b=SFdE5MYpLVSnTpFPyLVXmtyOV9d+he7xjqH3gu0+UVpJ7Afp66prI32RF+PUq4qop78y XxfA3d4dHWcQx6CCPfhfEWqO0DayGC+Q1Fnu2ltWJ+gi0eurg67WetHQMeNpSiXodumSG 0imnaZNa9bTbNFlztQ3GIsSCrS+MD3CZQcDYrwxCk1uGt5SFc8d+zqVyuTTKmKoW+3apL c/YeyN4GibMp6xFTLuE99IsTS1/L8RVZjnI5PMHBMcPEzyJs1Hrlx36jfQcq4syakr+1O TxCaHDjHXuhxX3MpdVCz6EnrFLk0/Iph7LeNWw5Zbpi+Rcp5EZ5+2jzDZAdwccGWZ7g== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; arc=none smtp.remote-ip=91.218.175.206 Received: from mta0.migadu.com (out-206.mta0.migadu.com [91.218.175.206]) (using TLSv1.3 with cipher TLS_AES_128_GCM_SHA256 (128/128 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E9B99606A31B for ; Tue, 27 Oct 2026 02:42:16 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org E9B99606A31B Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev X-Envelope-To: linux-kernel@vger.kernel.org DKIM-Signature: a=rsa-sha256; bh=gKD0g/74uU1V+yV//x6cgxOJGY2dkcadxe3i7Wn/Ix4=; c=simple/simple; d=linux.dev; h=from:to:subject:date:message-id:mime-version:content-type; s=key1; t=1793068934; v=1; x=1793673734; b=BPDTcX2tewPh8K3dVXk77Sr2df88g6qwlBGHCcIJY3cgczQc9q3TgPCZU0zQoUfVEtXc3CmX tuNHro/GUwUgT54IYyl3ySyWZE5zWURLdED8fziqb2ERCqw3rKnpEKsMvsBu+HOTArKrlw//X3C ZzAwsfGyndtrv+GVkwA6oSNI= X-Envelope-To: linux-kernel@vger.kernel.org Received: by mta12.migadu.com with ESMTPS id a11537fa0e8ad6d7; Tue, 27 Oct 2026 02:42:14 +0000 X-Mizu-Trace-ID: a11537fa0e8ad6d7 X-Migadu-Flow: FLOW_OUT Content-Type: text/plain; charset=us-ascii X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3901.100.1.1.12\)) Subject: Re: [PATCH] mm: memcontrol: fix NULL pointer deref in high_work_func() on dying memcg From: Muchun Song In-Reply-To: <20261023222313.3979455-1-tjmercier@google.com> Date: Tue, 27 Oct 2026 10:42:00 +0800 Cc: hannes@cmpxchg.org, mhocko@kernel.org, roman.gushchin@linux.dev, shakeel.butt@linux.dev, zhengqi.arch@bytedance.com, akpm@linux-foundation.org, cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org Content-Transfer-Encoding: quoted-printable Message-Id: References: <20261023222313.3979455-1-tjmercier@google.com> To: "T.J. Mercier" X-Mailer: Apple Mail (2.3901.100.1.1.12) > On Oct 24, 2026, at 06:23, T.J. Mercier wrote: >=20 > When memory.high is exceeded in non-task context, try_charge_memcg() > schedules memcg->high_work without taking a CSS reference on the = memcg, > and high_work_func() invokes reclaim_high() without pinning the memcg. >=20 > Before commit f1cf8d2f36dc ("mm: memcontrol: eliminate the problem of > dying memory cgroup for LRU folios"), memory usage above memory.high > kept the memcg's refcount non-zero. With reparenting, folios and = objcgs > charged to a child memcg are reparented to its parent in > mem_cgroup_css_offline(), leaving the offlined child's memory usage > permanently above its memory.high even after its last reference is > dropped. >=20 > cancel_work_sync(&memcg->high_work) only runs at the end of teardown = in > mem_cgroup_css_free(), so high_work_func() can start or still be = running > reclaim_high() after mem_cgroup_css_released() or shrink_many() has > called lru_gen_release_memcg() and unhashed lruvec->lrugen.list. This > causes lru_gen_rotate_memcg() to dereference a NULL pprev pointer. >=20 > Fix this by acquiring a reference with css_tryget_online() in > high_work_func() before calling reclaim_high(), and by resetting > memory.high and swap.high to PAGE_COUNTER_MAX in > mem_cgroup_css_offline() so that charges via references on an offlined > memcg do not trigger high_work on the dead child or skip checking live > ancestors in try_charge_memcg(). >=20 > Oops: general protection fault, probably for non-canonical address = 0xdffffc0000000000: 0000 [#1] SMP KASAN NOPTI > KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007] > CPU: 1 UID: 0 PID: 27 Comm: kworker/1:0 Tainted: G B W = 7.3.0-rc5-g2bba28e98578 #284 PREEMPT(full) <- Taint from = lru_gen_rotate_memcg() VM_WARN_ON_ONCE > Tainted: [B]=3DBAD_PAGE, [W]=3DWARN > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS = 1.17.0-debian-1.17.0-1 04/01/2014 > Workqueue: events high_work_func > RIP: 0010:lru_gen_rotate_memcg+0x1a3/0xe90 > Code: fd 7f 09 45 84 ed 0f 85 64 0c 00 00 48 89 cf c6 83 6a 07 00 00 = 00 48 c1 ef 03 88 83 69 07 00 00 48 b8 00 00 00 00 00 fc ff df <80> 3c = 07 00 0f 85 ec 0b 00 00 48 89 31 40 f6 c6 01 0f 85 4e 00 > RSP: 0018:ffa00000001ef4c8 EFLAGS: 00010046 > RAX: dffffc0000000000 RBX: ff1100000779e440 RCX: 0000000000000000 > RDX: 0000000000000001 RSI: ff1100000779b9b0 RDI: 0000000000000000 > RBP: ff110000bffd8e40 R08: 0000000000000000 R09: ff1100000779ebb8 > R10: ff1100000779ebb0 R11: 0000000000000246 R12: 0000000000000000 > R13: 0000000000000000 R14: ffa00000001efb30 R15: 0000000000000002 > FS: 0000000000000000(0000) GS:ff1100011c0e7000(0000) = knlGS:0000000000000000 > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > CR2: 000000000251ac88 CR3: 0000000009d6c000 CR4: 0000000000751ef0 > PKRU: 55555554 > Call Trace: > > ? read_ctrl_pos+0x22f/0x3f0 > shrink_lruvec+0x16cb/0x1b80 > ? ret_from_fork_asm+0x1a/0x30 > ? lapic_next_event+0x15/0x20 > ? clockevents_program_event+0x2bb/0x6f0 > ? lapic_next_event+0x15/0x20 > ? __pfx_shrink_lruvec+0x10/0x10 > ? __pfx_clockevents_program_event+0x10/0x10 > ? _raw_spin_lock+0x84/0xe0 > ? __pfx__raw_spin_lock+0x10/0x10 > ? kasan_save_stack+0x30/0x50 > ? psi_group_change+0x381/0x740 > ? __pfx_sched_clock_cpu+0x10/0x10 > ? save_fpregs_to_fpstate+0xc4/0x2c0 > ? finish_task_switch.isra.0+0x1de/0xac0 > ? __schedule+0xf62/0x42c0 > ? __pfx___schedule+0x10/0x10 > ? __pfx___irq_work_queue_local+0x10/0x10 > ? shrink_node+0x626/0x31e0 > shrink_node+0x626/0x31e0 > ? ring_buffer_unlock_commit+0x11d/0x5c0 > ? kswapd_clear_hopeless+0xa0/0x150 > ? __pfx_shrink_node+0x10/0x10 > ? 0xffffffffc0000095 > do_try_to_free_pages+0x255/0x13b0 > ? ring_buffer_unlock_commit+0x11d/0x5c0 > ? reclaim_high+0x10a/0x1b0 > try_to_free_mem_cgroup_pages+0x1f9/0x5a0 > ? __pfx_try_to_free_mem_cgroup_pages+0x10/0x10 > ? irqentry_exit+0xc6/0x670 > reclaim_high+0x10a/0x1b0 > ? __pfx_reclaim_high+0x10/0x10 > ? process_one_work+0x5ff/0x1030 > ? __pfx_high_work_func+0x10/0x10 > process_one_work+0x6e2/0x1030 > ? __pfx___schedule+0x10/0x10 > ? __pfx_process_one_work+0x10/0x10 > worker_thread+0x422/0xbb0 > ? __pfx_worker_thread+0x10/0x10 > kthread+0x2cd/0x3a0 > ? recalc_sigpending+0x15c/0x200 > ? __pfx_kthread+0x10/0x10 > ret_from_fork+0x368/0x590 > ? __pfx_ret_from_fork+0x10/0x10 > ? __switch_to+0x588/0xf50 > ? __pfx_kthread+0x10/0x10 > ret_from_fork_asm+0x1a/0x30 > >=20 > Fixes: f1cf8d2f36dc ("mm: memcontrol: eliminate the problem of dying = memory cgroup for LRU folios") Hi T.J., Thanks for fixing this. I have one question about the Fixes tag. The changelog says that, before f1cf8d2f36dc, memory usage above memory.high kept the memcg reference count non-zero. This is true for LRU folios: before that commit, each charged LRU folio held a CSS reference until it was uncharged. However, I wonder whether objcg-based kmem already provided a reachable path before f1cf8d2f36dc. In particular, BPF alloc_bulk() mostly runs from irq_work. It obtains a temporary memcg reference from a persistent objcg, uses = set_active_memcg() while performing __GFP_ACCOUNT allocations, and drops the temporary reference after the allocation. The charge can therefore queue memcg->high_work in non-task context without transferring a CSS reference to the work item. If this races with memcg offlining, objcg reparenting can redirect the objcg to the parent while the old child's page-counter usage remains above memory.high. Once the temporary CSS reference is dropped, mem_cgroup_css_released() can call lru_gen_release_memcg() before the queued high_work runs. The worker can then enter reclaim_high() and rotate an already removed lrugen.list. This path appears to have all the required pieces after: e4dde56cd208 ("mm: multi-gen LRU: per-node lru_gen_folio lists") Commit 6867c7a33206 later changed the release operation to hlist_nulls_del_init_rcu(), which explains the exact NULL pprev seen in the reported crash. Before that, the same invalid second deletion would instead encounter LIST_POISON2. So f1cf8d2f36dc clearly broadens the problem to ordinary LRU folios, but I am not very sure it is the first commit after which the underlying = race is reachable. Is there an additional lifetime guarantee that prevents the pre-f1cf objcg/kmem path above? If no, would the following be more appropriate? Fixes: e4dde56cd208 ("mm: multi-gen LRU: per-node lru_gen_folio lists") Thanks, Muchun