vfs, writeback: replace FS_CGROUP_WRITEBACK with SB_I_CGROUPWB

FS_CGROUP_WRITEBACK indicates whether a file_system_type supports
cgroup writeback; however, different super_blocks of the same
file_system_type may or may not support cgroup writeback depending on
filesystem options.  This patch replaces FS_CGROUP_WRITEBACK with a
per-super_block flag.

super_block->s_flags carries some internal flags in the high bits but
it's exposd to userland through uapi header and running out of space
anyway.  This patch adds a new field super_block->s_iflags to carry
kernel-internal flags.  It is currently only used by the new
SB_I_CGROUPWB flag whose concatenated and abbreviated name is for
consistency with other super_block flags.

ext2_fill_super() is updated to set SB_I_CGROUPWB.

v2: Added super_block->s_iflags instead of stealing another high bit
    from sb->s_flags as suggested by Christoph and Jan.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: linux-fsdevel@vger.kernel.org
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Jan Kara <jack@suse.cz>
Cc: linux-ext4@vger.kernel.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/fs/ext2/super.c b/fs/ext2/super.c
index 549219d44f1c..900e19cf9ef6 100644
--- a/fs/ext2/super.c
+++ b/fs/ext2/super.c
@@ -882,6 +882,7 @@ static int ext2_fill_super(struct super_block *sb, void *data, int silent)
         sb->s_flags = (sb->s_flags & ~MS_POSIXACL) |
                 ((EXT2_SB(sb)->s_mount_opt & EXT2_MOUNT_POSIX_ACL) ?
                  MS_POSIXACL : 0);
+        sb->s_iflags |= SB_I_CGROUPWB;
 
         if (le32_to_cpu(es->s_rev_level) == EXT2_GOOD_OLD_REV &&
             (EXT2_HAS_COMPAT_FEATURE(sb, ~0U) ||
@@ -1543,7 +1544,7 @@ static struct file_system_type ext2_fs_type = {
         .name           = "ext2",
         .mount          = ext2_mount,
         .kill_sb        = kill_block_super,
-        .fs_flags       = FS_REQUIRES_DEV | FS_CGROUP_WRITEBACK,
+        .fs_flags       = FS_REQUIRES_DEV,
 };
 MODULE_ALIAS_FS("ext2");