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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7686CC54EED for ; Sun, 29 Jan 2023 21:48:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235070AbjA2VsT (ORCPT ); Sun, 29 Jan 2023 16:48:19 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:55136 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229617AbjA2VsS (ORCPT ); Sun, 29 Jan 2023 16:48:18 -0500 Received: from dfw.source.kernel.org (dfw.source.kernel.org [139.178.84.217]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 34FFA1630C for ; Sun, 29 Jan 2023 13:48:17 -0800 (PST) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id C1F2D60C44 for ; Sun, 29 Jan 2023 21:48:16 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id BD6D9C433EF; Sun, 29 Jan 2023 21:48:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linux-foundation.org; s=korg; t=1675028896; bh=rhFf7uedNxdey8EUF7LX7Ea/PKGhbbJ0ekQViXUqr0g=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=rOHfC31UD2cUvJGxuNujV60wU4nBKmcOyjITFlE0VdHO+5d7EhJE4Nocfc/xpXTyV UKQeqUPQ3vb8vSXg2/FkEveP20SQUNe2UUF7ZamH4DaC+lsKYERxgU3PlLPwMWASMa DLz3xAeXJt7ieD/rQy9thZr3cRksofKSWFmYzUCY= Date: Sun, 29 Jan 2023 13:48:15 -0800 From: Andrew Morton To: Kefeng Wang Cc: Tejun Heo , Jens Axboe , Jan Kara , Shakeel Butt , Naoya Horiguchi , , , Ma Wupeng , Michal Hocko Subject: Re: [PATCH] mm: memcg: fix NULL pointer in mem_cgroup_track_foreign_dirty() Message-Id: <20230129134815.21083b65ef3ae4c3e7fae8eb@linux-foundation.org> In-Reply-To: <20230129024451.121590-1-wangkefeng.wang@huawei.com> References: <20230129024451.121590-1-wangkefeng.wang@huawei.com> X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, 29 Jan 2023 10:44:51 +0800 Kefeng Wang wrote: > As commit 18365225f044 ("hwpoison, memcg: forcibly uncharge LRU pages"), Merged in 2017. > hwpoison will forcibly uncharg a LRU hwpoisoned page, the folio_memcg > could be NULl, then, mem_cgroup_track_foreign_dirty_slowpath() could > occurs a NULL pointer dereference, let's do not record the foreign > writebacks for folio memcg is null in mem_cgroup_track_foreign() to > fix it. > > Reported-by: Ma Wupeng > Fixes: 97b27821b485 ("writeback, memcg: Implement foreign dirty flushing") Merged in 2019. > --- a/include/linux/memcontrol.h > +++ b/include/linux/memcontrol.h > @@ -1688,10 +1688,13 @@ void mem_cgroup_track_foreign_dirty_slowpath(struct folio *folio, > static inline void mem_cgroup_track_foreign_dirty(struct folio *folio, > struct bdi_writeback *wb) > { > + struct mem_cgroup *memcg; > + > if (mem_cgroup_disabled()) > return; > > - if (unlikely(&folio_memcg(folio)->css != wb->memcg_css)) > + memcg = folio_memcg(folio); > + if (unlikely(memcg && &memcg->css != wb->memcg_css)) > mem_cgroup_track_foreign_dirty_slowpath(folio, wb); > } Has this null deref actually been observed, or is this from code inspection? (This is why it's nice to include the Link: after a Reported-by!) Do we have any theories why this took so many years to surface? I'm confused about the mention of 18365225f044, but the Fixes: target is a different commit. Please explain this? Do you think the fix should be backported into earlier -stable kernels? If so, it will need some rework due to the subsequent folio conversion.