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>

1 files changed, 7 insertions, 4 deletions

diff --git a/fs/quota/quota.c b/fs/quota/quota.c
index d0efe302b1c1..3d31228082ea 100644
--- a/fs/quota/quota.c
+++ b/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;
         }