From: Amir Goldstein Subject: Re: [RFC][PATCH] sysrq: Emergency Remount R/O in reverse order Date: Thu, 7 May 2015 09:38:48 +0300 Message-ID: References: <20150430153130.GG12374@thunk.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Cc: linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, Oren Laadan , "Theodore Ts'o" To: viro@zeniv.linux.org.uk Return-path: In-Reply-To: <20150430153130.GG12374@thunk.org> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On Thu, Apr 30, 2015 at 6:31 PM, Theodore Ts'o wrote: > 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? Al, Any comment? Anybody else who dares to touch this code? Anybody who benefits from this change? Anybody who might be harmed from it? > > - 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 >> >