From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Wed, 4 Jul 2018 16:03:06 +0200 From: Michal Hocko To: Yang Shi Cc: "Theodore Y. Ts'o" , mgorman@techsingularity.net, adilger.kernel@dilger.ca, darrick.wong@oracle.com, dchinner@redhat.com, akpm@linux-foundation.org, linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/2] fs: ext4: use BUG_ON if writepage call comes from direct reclaim Message-ID: <20180704140306.GB22669@dhcp22.suse.cz> References: <1530591079-33813-1-git-send-email-yang.shi@linux.alibaba.com> <20180703103948.GB27426@thunk.org> <6c305241-d502-b8ea-a187-54c33e4ca692@linux.alibaba.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <6c305241-d502-b8ea-a187-54c33e4ca692@linux.alibaba.com> Sender: owner-linux-mm@kvack.org List-ID: On Tue 03-07-18 10:05:04, Yang Shi wrote: > > > On 7/3/18 3:39 AM, Theodore Y. Ts'o wrote: > > On Tue, Jul 03, 2018 at 12:11:18PM +0800, Yang Shi wrote: > > > direct reclaim doesn't write out filesystem page, only kswapd could do > > > it. So, if the call comes from direct reclaim, it is definitely a bug. > > > > > > And, Mel Gormane also mentioned "Ultimately, this will be a BUG_ON." In > > > commit 94054fa3fca1fd78db02cb3d68d5627120f0a1d4 ("xfs: warn if direct > > > reclaim tries to writeback pages"). > > > > > > Although it is for xfs, ext4 has the similar behavior, so elevate > > > WARN_ON to BUG_ON. > > > > > > And, correct the comment accordingly. > > > > > > Cc: Mel Gorman > > > Cc: "Theodore Ts'o" > > > Cc: Andreas Dilger > > > Signed-off-by: Yang Shi > > What's the upside of crashing the kernel if the file sytsem can handle it? > > I'm not sure if it is a good choice to let filesystem handle such vital VM > regression. IMHO, writing out filesystem page from direct reclaim context is > a vital VM bug. It means something is definitely wrong in VM. It should > never happen. Could you be more specific about the vital part please? Issuing writeback from the direct reclaim surely can be sub-optimal. But since we have quite a large stacks it shouldn't overflow immediately even for more complex storage setups. So what is the _vital_ bug here? -- Michal Hocko SUSE Labs