From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from zeniv.linux.org.uk ([195.92.253.2]:39312 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750783AbeCZFbw (ORCPT ); Mon, 26 Mar 2018 01:31:52 -0400 Date: Mon, 26 Mar 2018 06:31:51 +0100 From: Al Viro To: Dave Chinner Cc: linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH] fs: don't scan the inode cache before SB_ACTIVE is set Message-ID: <20180326053151.GO30522@ZenIV.linux.org.uk> References: <20180326043503.17828-1-david@fromorbit.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180326043503.17828-1-david@fromorbit.com> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Mon, Mar 26, 2018 at 03:35:03PM +1100, Dave Chinner wrote: > From: Dave Chinner > > We recently had an oops reported on a 4.14 kernel in > xfs_reclaim_inodes_count() where sb->s_fs_info pointed to garbage > and so the m_perag_tree lookup walked into lala land. > > We found a mount in a failed state, blocked on teh shrinker rwsem > here: > > mount_bdev() > deactivate_locked_super() > unregister_shrinker() > > Essentially, the machine was under memory pressure when the mount > was being run, xfs_fs_fill_super() failed after allocating the > xfs_mount and attaching it to sb->s_fs_info. It then cleaned up and > freed the xfs_mount, but the sb->s_fs_info field still pointed to > the freed memory. Hence when the superblock shrinker then ran > it fell off the bad pointer. > > This is reproduced by using the mount_delay sysfs control as added > in teh previous patch. It produces an oops down this path during the > stalled mount: > The problem is that the superblock shrinker is running before the > filesystem structures it depends on have been fully set up. i.e. > the shrinker is registered in sget(), before ->fill_super() has been > called, and the shrinker can call into the filesystem before > fill_super() does it's setup work. Wait a sec... How the hell does it get through trylock_super() before ->s_root is set and ->s_umount is unlocked?