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>

2 files changed, 4 insertions, 2 deletions

diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h
index dfce80869145..a13181a42b9a 100644
--- a/include/linux/backing-dev.h
+++ b/include/linux/backing-dev.h
@@ -260,7 +260,7 @@ static inline bool inode_cgwb_enabled(struct inode *inode)
 
         return bdi_cap_account_dirty(bdi) &&
                 (bdi->capabilities & BDI_CAP_CGROUP_WRITEBACK) &&
-                (inode->i_sb->s_type->fs_flags & FS_CGROUP_WRITEBACK);
+                (inode->i_sb->s_iflags & SB_I_CGROUPWB);
 }
 
 /**
diff --git a/include/linux/fs.h b/include/linux/fs.h
index b5e1dcfbc5e3..2c5e33a5b2af 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1241,6 +1241,8 @@ struct mm_struct;
 #define UMOUNT_NOFOLLOW 0x00000008      /* Don't follow symlink on umount */
 #define UMOUNT_UNUSED   0x80000000      /* Flag guaranteed to be unused */
 
+/* sb->s_iflags */
+#define SB_I_CGROUPWB   0x00000001      /* cgroup-aware writeback enabled */
 
 /* Possible states of 'frozen' field */
 enum {
@@ -1279,6 +1281,7 @@ struct super_block {
         const struct quotactl_ops       *s_qcop;
         const struct export_operations *s_export_op;
         unsigned long           s_flags;
+        unsigned long           s_iflags;       /* internal SB_I_* flags */
         unsigned long           s_magic;
         struct dentry           *s_root;
         struct rw_semaphore     s_umount;
@@ -1912,7 +1915,6 @@ struct file_system_type {
 #define FS_HAS_SUBTYPE          4
 #define FS_USERNS_MOUNT         8       /* Can be mounted by userns root */
 #define FS_USERNS_DEV_MOUNT     16 /* A userns mount does not imply MNT_NODEV */
-#define FS_CGROUP_WRITEBACK     32      /* Supports cgroup-aware writeback */
 #define FS_RENAME_DOES_D_MOVE   32768   /* FS will handle d_move() during rename() internally. */
         struct dentry *(*mount) (struct file_system_type *, int,
                        const char *, void *);