From: Amir Goldstein <amir@cellrox.com>
Subject: Re: [RFC][PATCH] sysrq: Emergency Remount R/O in reverse order
Date: Wed, 29 Apr 2015 21:34:47 +0300
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Cc: linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org,
	Oren Laadan <orenl@cellrox.com>, viro@zeniv.linux.org.uk
To: "Theodore Ts'o" <tytso@mit.edu>
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Hi Ted,

Did you see my query about emergency remount of loop mounted ext4?
Do you have any insights to share?

Thanks,
Amir.

On Fri, Apr 24, 2015 at 10:54 AM, Amir Goldstein <amir@cellrox.com> wrote:
> Hi Al,
>
> This patch is very naive. The fact that it tries to change a behavior
> that pre-dates modern git history suggests that I may be missing something.
>
> I am quite sure that the change of behavior is beneficial to my use case
> which involves loop mounts on Android, hence uploaded for Android:
> https://android-review.googlesource.com/148047
>
> I am not sure about the implications for other use cases, hence this RFC.
> Is there a reason to start remount from root fs, because later remounts
> may hang or take too long before some watchdog kicks in?
>
> Please enlighten me.
> Thanks,
>
> Amir.
>
> ---------- Forwarded message ----------
>
>
> 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 <amir@cellrox.com>
> Acked-by: Oren Laadan <orenl@cellrox.com>
> ---
>  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