linux-ext4 - [RFC][PATCH] sysrq: Emergency Remount R/O in reverse order

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Date:	Fri, 24 Apr 2015 10:54:35 +0300
From:	Amir Goldstein <amir@...lrox.com>
To:	viro@...iv.linux.org.uk
Cc:	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	"Theodore Ts'o" <tytso@....edu>, Oren Laadan <orenl@...lrox.com>
Subject: [RFC][PATCH] sysrq: Emergency Remount R/O in reverse order

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@...lrox.com>
Acked-by: Oren Laadan <orenl@...lrox.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
--
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