ceph: Fix incorrect statfs report for small quota

Problem:
The statfs reports incorrect free/available space for quota less then
CEPH_BLOCK size (4M).

Solution:
For quota less than CEPH_BLOCK size, smaller block size of 4K is used.
But if quota is less than 4K, it is decided to go with binary use/free
of 4K block. For quota size less than 4K size, report the
total=used=4K,free=0 when quota is full and total=free=4K,used=0
otherwise.

Signed-off-by: Kotresh HR <khiremat@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

Author: kotreshhr

fs/ceph/quota.c

@@ -494,10 +494,24 @@ bool ceph_quota_update_statfs(struct ceph_fs_client *fsc, struct kstatfs *buf)
 		if (ci->i_max_bytes) {
 			total = ci->i_max_bytes >> CEPH_BLOCK_SHIFT;
 			used = ci->i_rbytes >> CEPH_BLOCK_SHIFT;
+			/* For quota size less than 4MB, use 4KB block size */
+			if (!total) {
+				total = ci->i_max_bytes >> CEPH_4K_BLOCK_SHIFT;
+				used = ci->i_rbytes >> CEPH_4K_BLOCK_SHIFT;
+	                        buf->f_frsize = 1 << CEPH_4K_BLOCK_SHIFT;
+			}
 			/* It is possible for a quota to be exceeded.
 			 * Report 'zero' in that case
 			 */
 			free = total > used ? total - used : 0;
+			/* For quota size less than 4KB, report the
+			 * total=used=4KB,free=0 when quota is full
+			 * and total=free=4KB, used=0 otherwise */
+			if (!total) {
+				total = 1;
+				free = ci->i_max_bytes > ci->i_rbytes ? 1 : 0;
+	                        buf->f_frsize = 1 << CEPH_4K_BLOCK_SHIFT;
+			}
 		}
 		spin_unlock(&ci->i_ceph_lock);
 		if (total) {

fs/ceph/super.h

@@ -32,6 +32,7 @@
  * large volume sizes on 32-bit machines. */
 #define CEPH_BLOCK_SHIFT   22  /* 4 MB */
 #define CEPH_BLOCK         (1 << CEPH_BLOCK_SHIFT)
+#define CEPH_4K_BLOCK_SHIFT 12  /* 4 KB */
 
 #define CEPH_MOUNT_OPT_CLEANRECOVER    (1<<1) /* auto reonnect (clean mode) after blocklisted */
 #define CEPH_MOUNT_OPT_DIRSTAT         (1<<4) /* `cat dirname` for stats */