From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932762AbdBVOxa (ORCPT ); Wed, 22 Feb 2017 09:53:30 -0500 Received: from mx2.suse.de ([195.135.220.15]:47447 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932369AbdBVOxV (ORCPT ); Wed, 22 Feb 2017 09:53:21 -0500 Date: Wed, 22 Feb 2017 15:53:16 +0100 From: Michal Hocko To: Minchan Kim Cc: Andrew Morton , linux-kernel@vger.kernel.org, kernel-team@lge.com, Matthew Wilcox , stable@vger.kernel.org Subject: Re: [PATCH] mm: do not access page->mapping directly on page_endio Message-ID: <20170222145316.GL5753@dhcp22.suse.cz> References: <1487741964-17913-1-git-send-email-minchan@kernel.org> <20170222121100.GA7954@dhcp22.suse.cz> <20170222143517.GA18974@bbox> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170222143517.GA18974@bbox> User-Agent: Mutt/1.6.0 (2016-04-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed 22-02-17 23:35:17, Minchan Kim wrote: > On Wed, Feb 22, 2017 at 01:11:00PM +0100, Michal Hocko wrote: > > On Wed 22-02-17 14:39:24, Minchan Kim wrote: > > > With rw_page, page_endio is used for completing IO on a page > > > and it propagates write error to the address space if the IO > > > fails. The problem is it accesses page->mapping directly which > > > might be okay for file-backed pages but it shouldn't for > > > anonymous page. Otherwise, it can corrupt one of field from > > > anon_vma under us and system goes panic randomly. > > > > I was about to say that anonymous pages shouldn't hit that path because > > the end_swap_bio_write doesn call page_endio. But then I've noticed that > > No. For driver to support rw_page, every swap_writepage calls rw_page. > > swap_writepage > bdev_writepage > ops->rw_page Ohh, you are right, I have missed this option. I was looking at the normal swapout path which uses bio. > > zram does call this function. On a closer look, though, it doesn't seem > > to call it with err != 0 so it cannot hit this path. So I am wondering > > whether this actually fixes anything. Why it has been marked for stable? > > Look at other drivers to support rw_page, not zram, esp, brd. > They can be used for swap device and then can hit the case. > > In fact, I encountered the BUG during zram development(i.e., it doesn't > land to upstream) and it was really hard to figure it out because it made > random crash, sometime mmap_sem lockdep, sometime other places where > places never related to zram/zsmalloc, sometime not reproducible. > > When I consider how that bug is subtle and people do fast-swap test as brd, > it's worth to add stable mark, I think. Sure, could you add this to the changelog. Along with Fixes tag? I suspect it is dd6bd0d9c7db ("swap: use bdev_read_page() / bdev_write_page()") which has introduced this but I didn't look too close. The patch is trivially correct. -- Michal Hocko SUSE Labs