| Message ID | CAA2m6vfatWKS1CQFpaRbii2AXiZFvQUjVvYhGxWTSpz+2rxDyg@mail.gmail.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
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 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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 <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 > > -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Thu, Apr 30, 2015 at 6:31 PM, Theodore Ts'o <tytso@mit.edu> 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 <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 >> > -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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++;