xfs: xfs_seek_data() refinement with unwritten extents check up from page cache

index d78a746..3f91074 100644
@@ -1186,8 +1186,6 @@ xfs_seek_data(
 	struct inode		*inode = file->f_mapping->host;
 	struct xfs_inode	*ip = XFS_I(inode);
 	struct xfs_mount	*mp = ip->i_mount;
-	struct xfs_bmbt_irec	map[2];
-	int			nmap = 2;
 	loff_t			uninitialized_var(offset);
 	xfs_fsize_t		isize;
 	xfs_fileoff_t		fsbno;

@@ -1203,36 +1201,74 @@ xfs_seek_data(
 		goto out_unlock;
 	}
 
-	fsbno = XFS_B_TO_FSBT(mp, start);
-
 	/*
 	 * Try to read extents from the first block indicated
 	 * by fsbno to the end block of the file.
 	 */
+	fsbno = XFS_B_TO_FSBT(mp, start);
 	end = XFS_B_TO_FSB(mp, isize);
+	for (;;) {
+		struct xfs_bmbt_irec	map[2];
+		int			nmap = 2;
+		unsigned int		i;
 
-	error = xfs_bmapi_read(ip, fsbno, end - fsbno, map, &nmap,
-			       XFS_BMAPI_ENTIRE);
-	if (error)
-		goto out_unlock;
+		error = xfs_bmapi_read(ip, fsbno, end - fsbno, map, &nmap,
+				       XFS_BMAPI_ENTIRE);
+		if (error)
+			goto out_unlock;
 
-	/*
-	 * Treat unwritten extent as data extent since it might
-	 * contains dirty data in page cache.
-	 */
-	if (map[0].br_startblock != HOLESTARTBLOCK) {
-		offset = max_t(loff_t, start,
-			       XFS_FSB_TO_B(mp, map[0].br_startoff));
-	} else {
+		/* No extents at given offset, must be beyond EOF */
+		if (nmap == 0) {
+			error = ENXIO;
+			goto out_unlock;
+		}
+
+		for (i = 0; i < nmap; i++) {
+			offset = max_t(loff_t, start,
+				       XFS_FSB_TO_B(mp, map[i].br_startoff));
+
+			/* Landed in a data extent */
+			if (map[i].br_startblock == DELAYSTARTBLOCK ||
+			    (map[i].br_state == XFS_EXT_NORM &&
+			     !isnullstartblock(map[i].br_startblock)))
+				goto out;
+
+			/*
+			 * Landed in an unwritten extent, try to search data
+			 * from page cache.
+			 */
+			if (map[i].br_state == XFS_EXT_UNWRITTEN) {
+				if (xfs_find_get_desired_pgoff(inode, &map[i],
+							DATA_OFF, &offset))
+					goto out;
+			}
+		}
+
+		/*
+		 * map[0] is hole or its an unwritten extent but
+		 * without data in page cache.  Probably means that
+		 * we are reading after EOF if nothing in map[1].
+		 */
 		if (nmap == 1) {
 			error = ENXIO;
 			goto out_unlock;
 		}
 
-		offset = max_t(loff_t, start,
-			       XFS_FSB_TO_B(mp, map[1].br_startoff));
+		ASSERT(i > 1);
+
+		/*
+		 * Nothing was found, proceed to the next round of search
+		 * if reading offset not beyond or hit EOF.
+		 */
+		fsbno = map[i - 1].br_startoff + map[i - 1].br_blockcount;
+		start = XFS_FSB_TO_B(mp, fsbno);
+		if (start >= isize) {
+			error = ENXIO;
+			goto out_unlock;
+		}
 	}
 
+out:
 	if (offset != file->f_pos)
 		file->f_pos = offset;