From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 7BC39C44529 for ; Mon, 20 Jul 2026 09:12:04 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 375FC6B0088; Mon, 20 Jul 2026 05:12:03 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 34D9F6B008A; Mon, 20 Jul 2026 05:12:03 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 263E56B008C; Mon, 20 Jul 2026 05:12:03 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0016.hostedemail.com [216.40.44.16]) by kanga.kvack.org (Postfix) with ESMTP id 0359B6B0088 for ; Mon, 20 Jul 2026 05:12:02 -0400 (EDT) Received: from smtpin08.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay08.hostedemail.com (Postfix) with ESMTP id 7E8A914071F for ; Mon, 20 Jul 2026 09:12:02 +0000 (UTC) X-FDA: 85008588084.08.8DB619E Received: from lgeamrelo11.lge.com (lgeamrelo11.lge.com [156.147.23.51]) by imf12.hostedemail.com (Postfix) with ESMTP id 6C6D740005 for ; Mon, 20 Jul 2026 09:11:59 +0000 (UTC) Authentication-Results: imf12.hostedemail.com; dkim=none; spf=pass (imf12.hostedemail.com: domain of youngjun.park@lge.com designates 156.147.23.51 as permitted sender) smtp.mailfrom=youngjun.park@lge.com; dmarc=pass (policy=none) header.from=lge.com ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1784538720; b=I63rtx+/lQQeYNU+MmJKbywCHQmooy4SvFIa8iecVeoQC1hNpuE7gNhWQHWU5Pb1xL2tNf ZlOUXPzep3TP10kHUKcljstJpvSeo3HNKPlYhDGALZqesChdtT0WQ0fSkKTYpoIjhboAnr M9oQBDuynJfL7wF3UefcZZrQwNmjwbE= ARC-Authentication-Results: i=1; imf12.hostedemail.com; dkim=none; spf=pass (imf12.hostedemail.com: domain of youngjun.park@lge.com designates 156.147.23.51 as permitted sender) smtp.mailfrom=youngjun.park@lge.com; dmarc=pass (policy=none) header.from=lge.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1784538720; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=PnmzFgZfhE1Jp7fyBRSesXFLlfdtMYnvxjhosjCEu4c=; b=NutTv6pn+QPjFhX0fw+Yohwcm9W9GuS4GkycV6WNozjqkFE/7Fu+ZTVbNW9WUxb0mmeZ9o 0F3Ub9q9LYtygVhIbAPcEPgB6nemXy8kt3cJqwJtdV/Xa4Rzcqeo2Hd7Cf7EShgjoqvprn yDDGd+YS0wGLRDQhwMPu8us5xl4wW+Y= Received: from unknown (HELO lgeamrelo01.lge.com) (156.147.1.125) by 156.147.23.51 with ESMTP; 20 Jul 2026 18:11:54 +0900 X-Original-SENDERIP: 156.147.1.125 X-Original-MAILFROM: youngjun.park@lge.com Received: from unknown (HELO yjaykim-PowerEdge-T330) (10.177.112.156) by 156.147.1.125 with ESMTP; 20 Jul 2026 18:11:54 +0900 X-Original-SENDERIP: 10.177.112.156 X-Original-MAILFROM: youngjun.park@lge.com Date: Mon, 20 Jul 2026 18:11:54 +0900 From: Youngjun Park To: Kemeng Shi Cc: chrisl@kernel.org, kasong@tencent.com, nphamcs@gmail.com, baoquan.he@linux.dev, baohua@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/4] mm, swap: Fix potential NULL dereference when trying a sleep table allocation Message-ID: References: <20260720071342.50742-1-shikemeng@huaweicloud.com> <20260720071342.50742-2-shikemeng@huaweicloud.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20260720071342.50742-2-shikemeng@huaweicloud.com> X-Rspam-User: X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 6C6D740005 X-Stat-Signature: osaw8t4qpmskxqiakcn67srtjs57x1ut X-HE-Tag: 1784538719-648944 X-HE-Meta: U2FsdGVkX1++pVOAxXKUo519Sssrjv3DoTgzCASnGzq62RrSRkhV27x9bKe6tNfhxIYdLdFBhgjtsGcg/KYjNrxdVb/rVsYLNQyDZCFKhiAHWEw8FTTJDY+nv8pKMVFSmE5sqZAJeCVvq8V/xB/YdCKPUw6YXzgyx/zsUpNPvoyVcezni9b+Eidkm2bTZrrkNeNKkXyHSpm4diI4L0G0qvzHxfAudJsJHL+HK9PZDXqfppQNb/S4OLMLFbm8WPL7fQ3/N5IQPd4B09WIAMykZ4nF1Acg8pr6199lTSPmiHH+dlP6u4FVpBWjdVuPwNjqBAdKdauBpFhhyHg884KtyW7trZOpr26fv0SdaPQZIyrebCsd5zBjVFIw98s8aWKEnMHRIZIQThVIs2NgYRfsVdc7RoprEK8DEGT7jV69Jo2Q53FsUKGY30B9ksQqIQYawXEbWKS2EHN3/QeXBAjQY0EpsClabA6q9KcA3R+vN9TsWdS6PCHAZmEbII0TaWPzV+KiqnkPHVvfwV5towQdSUS0u1cScs9doV3rpUCz8UhEVilSKfzTOhUtIrfuLePevQ3Mz0GlWk37xHDHu9SZL73kQjHyiY+UxDaE0CgtSVj0PT3lsTzm6z0d0vDcfFV4B0jPRSshAtx3CE4pCq2cUsO+mgJbHVURk6iuxa44mOft2KfFgJL4gMbpMtsgRo+JYhOaU/zqOroSXrv75RDkBvAOdGjmp2IjR9RZblUODQXRuLI8EJZs31W2hK1lbE2WolWfYYU6dG8fTV/6PBhI5GLVeOd+6aGCVLoDc9NqdXiLODIfgUM0YcF+MjeDtAy/9E/Vps7wEXGTKA87Ic/Lcy6UbANTKXjzbhJPehXmZ6J+/7YaJeXpphFgucBbG7yZOjBN7iDVHntWlIm7Z1iijTu1ZTwGNSZOCJdHsiLkMGXjMzqJC4rCSMhKhG9VzGnsosRvD5NxoOTSpDj9w2Z B0Tw2wWi 5KVi2lbGn97xwifsO60QESNmchPsJIjMtbAlMcq4GLwiOTsuOqpUYRlZk8+88rPwR8DPzkTK74MfDeYnM9SsUIUNokqKW6Lxu5vFOWK/ttVyfY7AbDMgWdisT/afnNZ4Ue4bjlYUBCR0mCvO/EgjsEyTPHZc0KXz5rXTGkX/+fMk3TXKN+gMVIEzqwtkrI+0czfGQiuuMrNhIzqLGf/xSmn3iWG9Ta/2i8377kQw1cU5JRqU= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Jul 20, 2026 at 03:13:39PM +0800, Kemeng Shi wrote: Hello Kemeng Shi, Good catch! it indeed looks like a possible race condition (though I haven't verified it at runtime either). > The root cause of this issue is because multi-tables are updated in non > atomic context. To be more specific, the issue could be triggerred as > following: Here the cluster is isolated (CLUSTER_FLAG_NONE). > swap_alloc_fast swap_cluster_populate() > /* Try a sleep allocation */ > spin_unlock(&ci->lock); And from here, the table becomes visible, > swap_cluster_alloc_table() > rcu_assign_pointer(ci->table, table); All entry on this cluster is freed(e.g process dead) , so this might be the free cluster. > ci = swap_cluster_lock(si, offset) > cluster_is_usable(ci, order） Since it's a normal cluster (CLUSTER_FLAG_NONE) and the table exists, it passes here... > if (!cluster_table_is_alloced(ci)) // ok > alloc_swap_scan_cluster() > cluster_scan_range() > __swap_table_get() > > /* free table when more table allocation fails */ > ci->memcg_table = kzalloc_obj(*ci->memcg_table, > gfp); > if (!ci->memcg_table) > swap_cluster_free_table() Nullified > rcu_assign_pointer(ci->table, NULL); Now it happens. > table = rcu_dereference_check(ci->table, lockdep_is_held(&ci->lock)); > atomic_long_read(&table[off]); // NULL dereference > > Fix the issue by updating allocated tables in atomic context. > > Fixes: 2fe7a6f5024b8 ("mm/memcg, swap: store cgroup id in cluster table directly") > Signed-off-by: Kemeng Shi > --- > mm/swapfile.c | 21 ++++++++++++++++++++- > 1 file changed, 20 insertions(+), 1 deletion(-) > > diff --git a/mm/swapfile.c b/mm/swapfile.c > index 615d90867111..d29062d9c3cd 100644 > --- a/mm/swapfile.c > +++ b/mm/swapfile.c > @@ -490,6 +490,20 @@ static int swap_cluster_alloc_table(struct swap_cluster_info *ci, gfp_t gfp) > return 0; > } > > +static void swap_cluster_copy_table(struct swap_cluster_info *d_ci, > + struct swap_cluster_info *s_ci) > +{ > + rcu_assign_pointer(d_ci->table, rcu_access_pointer(s_ci->table)); > + > +#ifdef CONFIG_MEMCG > + d_ci->memcg_table = s_ci->memcg_table; > +#endif > + > +#if !SWAP_TABLE_HAS_ZEROFLAG > + d_ci->zero_bitmap = s_ci->zero_bitmap; > +#endif > +} > + > /* > * Sanity check to ensure nothing leaked, and the specified range is empty. > * One special case is that bad slots can't be freed, so check the number of > @@ -527,6 +541,7 @@ static struct swap_cluster_info * > swap_cluster_populate(struct swap_info_struct *si, > struct swap_cluster_info *ci) > { > + struct swap_cluster_info tmp_ci; > int ret; IMHO, How about resolving everything inside swap_cluster_alloc_table() instead? We could consider the following two things. - Assign the table at the very end inside swap_cluster_alloc_table(). - Handle the table freeing properly if subsequent allocations fail. Rather than allocating a temp_ci (which adds special handling for this case), wouldn't it be better to maintain the original intention of the swap_cluster_alloc_table() function? Although this is not a hot path and table allocation will usually succeed with the ATOMIC allocator, this approach would also save the memcpy overhead and stack memory usage. (Another option might be checking memcg_table and zero_bitmap inside cluster_table_is_alloced(). However, I personally dislike this idea as it might introduce side effects regarding its coverage.) Your current approach is also a good direction, but please review this idea as well :) Thanks, Youngjun