From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out30-100.freemail.mail.aliyun.com (out30-100.freemail.mail.aliyun.com [115.124.30.100]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B6F14412294 for ; Wed, 2 Sep 2026 09:16:36 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=115.124.30.100 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788340601; cv=none; b=SvkkBbf7s5YweFASNrTiimvWKQSMR+IvgwwBlNI2d0LorA+lbM2U5OVXI+5I7RM9Vrkj8gTn1KpoQ/iaxN4O8eKTYsB2zcUb0a/X/QWEEpMyQd+Z1GmWGxEuJAjOoUyTjMo7zDyqA5FMk9EkoRu2fbraiWXsw8ZAj1jwd3JKJ1c= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788340601; c=relaxed/simple; bh=EUQCKHHWENRDaX9sisQeocToD6AnEwmF3JspryVA8D4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=YShP6Z0u2VSU8Li/vZvzsrl7PpW6mU6sdvTSGywKz+/joEk9MfDvo31N99DVzKD67KhQx5udVUH36hbIZ4ltUvRhK2kwqjPaUBJLZtfe+E2i2bcYd8x3QELO0chTy85q49nSFele6ikbWscqItU2haFJo1lgj7LfzPbG7uNs0Uc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com; spf=pass smtp.mailfrom=linux.alibaba.com; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b=ySNfnkl0; arc=none smtp.client-ip=115.124.30.100 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.alibaba.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.alibaba.com header.i=@linux.alibaba.com header.b="ySNfnkl0" DKIM-Signature:v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.alibaba.com; s=default; t=1788340587; h=Message-ID:Date:MIME-Version:Subject:To:From:Content-Type; bh=/xWKMGzD5oNg8UWITXcf+NlpDdHUkl8dRMjiWqCZA7k=; b=ySNfnkl0BjONp1HE0S/oQTQ4zqT6vr7l2OtO2InGDOfMn+24quHnIA1c96J8LWLh6xcudwZqLKqjvK8fo+TtTuJ4E36d6taZhr0GBjAqJmlC75bkvls4ivK49dlyVplz7t/D1kED93Q28ZPoDPk9Dgry/CuD4crGrCS6bb4JqW0= X-Alimail-AntiSpam:AC=PASS;BC=-1|-1;BR=01201311R591e4;CH=green;DM=||false|;DS=||;FP=0|-1|-1|-1|0|-1|-1|-1;HT=maildocker-contentspam033045133197;MF=baolin.wang@linux.alibaba.com;NM=1;PH=DS;RN=9;SR=0;TI=SMTPD_---0XACEJlP_1788340586; Received: from 30.74.144.115(mailfrom:baolin.wang@linux.alibaba.com fp:SMTPD_---0XACEJlP_1788340586 cluster:ay36) by smtp.aliyun-inc.com; Wed, 02 Sep 2026 17:16:27 +0800 Message-ID: Date: Wed, 2 Sep 2026 17:16:25 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/list_lru: disable memcg awareness under cgroup_disable=memory To: Qinyun Tan , Andrew Morton Cc: Dave Chinner , Qi Zheng , Roman Gushchin , Muchun Song , Xunlei Pang , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260902032840.30250-1-qinyuntan@linux.alibaba.com> From: Baolin Wang In-Reply-To: <20260902032840.30250-1-qinyuntan@linux.alibaba.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 9/2/26 11:28 AM, Qinyun Tan wrote: > __list_lru_init() only collapses a memcg-aware list_lru into plain > per-node lists when kmem accounting is disabled > (cgroup.memory=nokmem). When the memory controller is disabled > entirely (cgroup_disable=memory), mem_cgroup_kmem_disabled() is > false, so the lru stays memcg aware even though no object will ever > be charged to a memcg. > > This is more than a semantic inconsistency. > folio_memcg_list_lru_alloc() trusts list_lru_memcg_aware() and > dereferences the folio's memcg, which is always NULL with the > controller disabled. The only mainline caller, > folio_memcg_alloc_deferred(), papers over this with an explicit > mem_cgroup_disabled() check. The shmem unused-huge shrinker > conversion ("mm: shmem: make unused huge shrinker memcg aware") adds > a second caller without such a guard, so booting with > cgroup_disable=memory and writing to a huge=always tmpfs oopses: > > BUG: unable to handle page fault for address: 0000000000000488 > RIP: 0010:folio_memcg_list_lru_alloc+0x41/0xf0 > Call Trace: > > shmem_get_folio_gfp+0x1cd/0x7c0 > shmem_write_begin+0x5d/0x100 > generic_perform_write+0x89/0x2a0 > shmem_file_write_iter+0x82/0x90 > vfs_write+0x256/0x410 > ksys_write+0x61/0xe0 > do_syscall_64+0x8d/0x460 > entry_SYSCALL_64_after_hwframe+0x76/0x7e > > The faulting address is the offset of mem_cgroup->kmemcg_id, > dereferenced on a NULL memcg in memcg_list_lru_allocated(): > > folio_memcg_list_lru_alloc() > list_lru_memcg_aware() <- true, only nokmem checked > memcg = folio_memcg(folio) <- NULL > memcg_list_lru_allocated(memcg, lru) > memcg->kmemcg_id <- NULL pointer dereference > > Check mem_cgroup_disabled() in __list_lru_init() so that all > list_lrus fall back to plain per-node lists when the controller is > disabled, matching what the shrinker side already does > (shrinker_memcg_alloc() bails out on mem_cgroup_disabled()). This > makes the mem_cgroup_disabled() check in callers unnecessary rather > than mandatory. > > Signed-off-by: Qinyun Tan > --- > Applies on top of mm-new. No Fixes: tag since the commit that makes > the crash reachable ("mm: shmem: make unused huge shrinker memcg > aware") is only in mm-new; no stable backport is needed either. > > Reproducer, on mm-new booted with cgroup_disable=memory > (CONFIG_MEMCG=y, CONFIG_TRANSPARENT_HUGEPAGE=y): > > # mount -t tmpfs -o huge=always tmpfs /mnt > # echo x > /mnt/f > > Without this patch the write oopses immediately as shown above: the > freshly allocated huge folio extends beyond i_size, so > shmem_get_folio_gfp() queues the inode via shmem_unused_huge_add() > -> folio_memcg_list_lru_alloc(), which dereferences the NULL > folio_memcg(). > > With this patch the same steps run cleanly: the lru falls back to > plain per-node lists and folio_memcg_list_lru_alloc() returns early. > From code inspection the rest of the shmem path handles the NULL > objcg fine (obj_cgroup_memcg() and obj_cgroup_put() are NULL-safe, > and list_lru_add() with a NULL memcg lands on the per-node list), > but I have not exercised the shrinker reclaim itself under > cgroup_disable=memory. > > Discussion: https://lore.kernel.org/linux-mm/20260901115104.2944996-1-qinyuntan@linux.alibaba.com/ > > mm/list_lru.c | 7 ++++++- > 1 file changed, 6 insertions(+), 1 deletion(-) > > diff --git a/mm/list_lru.c b/mm/list_lru.c > index 36662d02ff963..f8be119351cca 100644 > --- a/mm/list_lru.c > +++ b/mm/list_lru.c > @@ -671,7 +671,12 @@ int __list_lru_init(struct list_lru *lru, bool memcg_aware, struct shrinker *shr > else > lru->shrinker_id = -1; > > - if (mem_cgroup_kmem_disabled()) > + /* > + * With the memory controller disabled entirely, no object is ever > + * charged to a memcg, so collapse to plain per-node lists just > + * like under nokmem. > + */ These comments seem useless, as the code already explains itself. > + if (mem_cgroup_disabled() || mem_cgroup_kmem_disabled()) > memcg_aware = false; > #endif > Since __list_lru_init() is also exported, I think this looks reasonable to me. With comments removed, Reviewed-by: Baolin Wang