From: Theodore Ts'o Subject: Re: [RFC][PATCH] sysrq: Emergency Remount R/O in reverse order Date: Thu, 30 Apr 2015 11:31:30 -0400 Message-ID: <20150430153130.GG12374@thunk.org> References: Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, Oren Laadan , viro@zeniv.linux.org.uk To: Amir Goldstein Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On Wed, Apr 29, 2015 at 09:33:45PM +0300, Amir Goldstein wrote: > Hi Ted, > > Did you see my query about emergency remount of loop mounted ext4? > Do you have any insights to share? Your patch makes sense to me. Doing the remount-ro in reverse order that file systems were mounted sounds like the right thing to do. I can imagine trying to make ext4 handle the situation where it is trying to do an unmount, or remount r/o, and the device has gone read-only in a similar situation as one where the device has disappeared altogether, but it's not going to solve all possible issues. Trying to unmount or remount r/o the most recently mounted file system first is simpler and will solve more problems. Al? - Ted > > > > This change fixes a problem where reboot on Android panics the kernel > > almost every time when file systems are mounted over loop devices. > > > > Android reboot command does: > > - sync > > - echo u > /proc/sysrq-trigger > > - syscall_reboot > > > > The problem is with sysrq emergency remount R/O trying to remount-ro > > in wrong order. > > since /data is re-mounted ro before loop devices, loop device > > remount-ro fails to flush the journal and panics the kernel: > > > > EXT4-fs (loop0): Remounting filesystem read-only > > EXT4-fs (loop0): previous I/O error to superblock detected > > loop: Write error at byte offset 0, length 4096. > > Buffer I/O error on device loop0, logical block 0 > > lost page write due to I/O error on loop0 > > Kernel panic - not syncing: EXT4-fs panic from previous error > > > > The fix is quite simple. In do_emergency_remount(), use > > list_for_each_entry_reverse() on sb list instead of list_for_each_entry(). > > It makes a lot of sense to umount the file systems in reverse order in > > which they were added to sb list. > > > > Signed-off-by: Amir Goldstein > > Acked-by: Oren Laadan > > --- > > fs/super.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/fs/super.c b/fs/super.c > > index 2b7dc90..f1315e0 100644 > > --- a/fs/super.c > > +++ b/fs/super.c > > @@ -774,7 +774,7 @@ static void do_emergency_remount(struct work_struct > > *work) > > struct super_block *sb, *p = NULL; > > > > spin_lock(&sb_lock); > > - list_for_each_entry(sb, &super_blocks, s_list) { > > + list_for_each_entry_reverse(sb, &super_blocks, s_list) { > > if (hlist_unhashed(&sb->s_instances)) > > continue; > > sb->s_count++; > > -- > > 1.8.2 > >