From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl1-f200.google.com (mail-pl1-f200.google.com [209.85.214.200]) by kanga.kvack.org (Postfix) with ESMTP id 5B4C28E0001 for ; Wed, 19 Dec 2018 14:00:29 -0500 (EST) Received: by mail-pl1-f200.google.com with SMTP id p3so15319612plk.9 for ; Wed, 19 Dec 2018 11:00:29 -0800 (PST) Received: from mga18.intel.com (mga18.intel.com. [134.134.136.126]) by mx.google.com with ESMTPS id 26si16993336pgu.190.2018.12.19.11.00.27 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 19 Dec 2018 11:00:28 -0800 (PST) Subject: Re: [RFC PATCH 1/2] mm: swap: check if swap backing device is congested or not References: <1545115948-25467-1-git-send-email-yang.shi@linux.alibaba.com> <6a51d47a-b87f-b0f1-4dae-843730dba698@linux.intel.com> <03462367-3f75-4907-8c5e-526a919e5cf3@linux.alibaba.com> <606b73ed-da62-fdd1-71da-c4de7a02e837@linux.alibaba.com> <2df52dc3-ae66-0ab8-459e-49f49eaa569c@linux.alibaba.com> From: Tim Chen Message-ID: <50e14a30-97bb-06c6-ae6a-74e6dc827713@linux.intel.com> Date: Wed, 19 Dec 2018 11:00:27 -0800 MIME-Version: 1.0 In-Reply-To: <2df52dc3-ae66-0ab8-459e-49f49eaa569c@linux.alibaba.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: owner-linux-mm@kvack.org List-ID: To: Yang Shi , ying.huang@intel.com, tim.c.chen@intel.com, minchan@kernel.org, Andrew Morton Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org On 12/19/18 10:40 AM, Yang Shi wrote: > > >>>> I don't think your dereference inode = si->swap_file->f_mapping->host >>>> is always safe.  You should do it only when (si->flags & SWP_FS) is true. >>> Do you mean it is not safe for swap partition? >> The f_mapping may not be instantiated.  It is only done for SWP_FS. > > Really? I saw the below calls in swapon: > > swap_file = file_open_name(name, O_RDWR|O_LARGEFILE, 0); > ... > p->swap_file = swap_file; > mapping = swap_file->f_mapping; > inode = mapping->host; > ... > > Then the below code manipulates the inode. > > And, trace shows file_open_name() does call blkdev_open if it is turning block device swap on. And, blkdev_open() would return instantiated address_space and inode. > > Am I missing something? > I was trying to limit the congestion logic for block devices backed swap. So the check I had in mind should really be "si->flags & SWP_BLKDEV" instead of si->flags & SWP_FS. I was concerned that there could be other use cases where the inode dereference is invalid. Looking at the code a bit more, looks like swap_cluster_readahead is not used for other special case swap usage (like page migration). So you would a proper swapfile and inode here. But I think it is still a good idea to have a check for SWP_BLKDEV in si->flags. Thanks. Tim