From mboxrd@z Thu Jan 1 00:00:00 1970 From: Amir Goldstein Subject: [RFC][PATCH] sysrq: Emergency Remount R/O in reverse order Date: Fri, 24 Apr 2015 10:54:35 +0300 Message-ID: Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Cc: linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, "Theodore Ts'o" , Oren Laadan To: viro@zeniv.linux.org.uk Return-path: Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org 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 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