quota: clean up Q_XQUOTASYNC

Currently Q_XQUOTASYNC calls into the quota_sync method, but XFS does something
entirely different in it than the rest of the filesystems.  xfs_quota which
calls Q_XQUOTASYNC expects an asynchronous data writeout to flush delayed
allocations, while the "VFS" quota support wants to flush changes to the quota
file.

So make Q_XQUOTASYNC call into the writeback code directly and make the
quota_sync method optional as XFS doesn't need in the sense expected by the
rest of the quota code.

GFS2 was using limited XFS-style quota and has a quota_sync method fitting
neither the style used by vfs_quota_sync nor xfs_fs_quota_sync.  I left it
in for now as per discussion with Steve it expects to be called from the
sync path this way.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/quota/quota.c

@@ -18,6 +18,7 @@
 #include <linux/capability.h>
 #include <linux/quotaops.h>
 #include <linux/types.h>
+#include <linux/writeback.h>
 #include <net/netlink.h>
 #include <net/genetlink.h>
 
@@ -52,7 +53,7 @@ void sync_quota_sb(struct super_block *sb, int type)
 {
 	int cnt;
 
-	if (!sb->s_qcop->quota_sync)
+	if (!sb->s_qcop || !sb->s_qcop->quota_sync)
 		return;
 
 	sb->s_qcop->quota_sync(sb, type);
@@ -318,9 +319,11 @@ static int do_quotactl(struct super_block *sb, int type, int cmd, qid_t id,
 	case Q_XGETQUOTA:
 		return quota_getxquota(sb, type, id, addr);
 	case Q_XQUOTASYNC:
-		if (!sb->s_qcop->quota_sync)
-			return -ENOSYS;
-		return sb->s_qcop->quota_sync(sb, type);
+		/* caller already holds s_umount */
+		if (sb->s_flags & MS_RDONLY)
+			return -EROFS;
+		writeback_inodes_sb(sb);
+		return 0;
 	default:
 		return -EINVAL;
 	}

fs/xfs/linux-2.6/xfs_quotaops.c

@@ -43,20 +43,6 @@ xfs_quota_type(int type)
 	}
 }
 
-STATIC int
-xfs_fs_quota_sync(
-	struct super_block	*sb,
-	int			type)
-{
-	struct xfs_mount	*mp = XFS_M(sb);
-
-	if (sb->s_flags & MS_RDONLY)
-		return -EROFS;
-	if (!XFS_IS_QUOTA_RUNNING(mp))
-		return -ENOSYS;
-	return -xfs_sync_data(mp, 0);
-}
-
 STATIC int
 xfs_fs_get_xstate(
 	struct super_block	*sb,
@@ -151,7 +137,6 @@ xfs_fs_set_xquota(
 }
 
 const struct quotactl_ops xfs_quotactl_operations = {
-	.quota_sync		= xfs_fs_quota_sync,
 	.get_xstate		= xfs_fs_get_xstate,
 	.set_xstate		= xfs_fs_set_xstate,
 	.get_xquota		= xfs_fs_get_xquota,

include/linux/quotaops.h

@@ -22,7 +22,7 @@ static inline struct quota_info *sb_dqopt(struct super_block *sb)
 void sync_quota_sb(struct super_block *sb, int type);
 static inline void writeout_quota_sb(struct super_block *sb, int type)
 {
-	if (sb->s_qcop->quota_sync)
+	if (sb->s_qcop && sb->s_qcop->quota_sync)
 		sb->s_qcop->quota_sync(sb, type);
 }