No SysRq remount because sb->s_bdev is NULL

Hi,

During a SysRq emergency remount Btrfs mounts are not remounted. I tracked the issue down to this line in do_emergency_remount() in fs/super.c:
if (sb->s_root && sb->s_bdev && (sb->s_flags & MS_BORN) &&

	!(sb->s_flags & MS_RDONLY))

s_bdev is NULL for Btrfs super blocks and subsequently do_remount_sb() is never called. I couldn't think of a solution, besides resorting to an ugly strcmp(sb->s_type->name, "btrfs"), without adding a field to struct super_block or similar changes.
Thoughts?

Flo
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Quoted 9 lines · Florian Albrechtskirchinger

During a SysRq emergency remount Btrfs mounts are not remounted. I tracked the issue down to this line in do_emergency_remount() in fs/super.c:
if (sb->s_root && sb->s_bdev && (sb->s_flags & MS_BORN) &&

	!(sb->s_flags & MS_RDONLY))

s_bdev is NULL for Btrfs super blocks and subsequently do_remount_sb() is never called. I couldn't think of a solution, besides resorting to an ugly strcmp(sb->s_type->name, "btrfs"), without adding a field to struct super_block or similar changes.
Thoughts?

Just a first thought. Is there a possibility to write a dummy value into sb->s_bdev for btrfs super blocks. Thus it will not be NULL and everything in do_emergency_remount() in fs/super.c will work as wanted.

s_bdev is a valid block device would fail.

Quoted 2 lines · On Saturday, July 28, 2012 15:46:50 Andreas Philipp wrote:

Quoted 1 line · Florian Albrechtskirchinger

During a SysRq emergency remount Btrfs mounts are not remounted. I

Quoted 4 lines

Quoted 4 lines

the issue down to this line in do_emergency_remount() in fs/super.c:
if (sb->s_root && sb->s_bdev && (sb->s_flags & MS_BORN) && !(sb->s_flags & MS_RDONLY))
s_bdev is NULL for Btrfs super blocks and subsequently

Quoted 1 line

Quoted 1 line

never called. I couldn't think of a solution, besides resorting to

Quoted 7 lines

Quoted 3 lines

strcmp(sb->s_type->name, "btrfs"), without adding a field to struct super_block or similar changes.
Thoughts?

Just a first thought. Is there a possibility to write a dummy value into sb->s_bdev for btrfs super blocks. Thus it will not be NULL and everything in do_emergency_remount() in fs/super.c will work as wanted.

Then other code paths testing sb->s_bdev for NULL and assuming a

Quoted 1 line

s_bdev is a valid block device would fail.

Quoted 1 line

Flo