Introduce SEEK_DATA/SEEK_HOLE to XFS V5

[Mark Tinguely]

xfs_has_unwritten_buffer() always returns the offset of the first
dirty unwritten page. This can cause xfs_seek_data() and xfs_seek_hole()
to give the wrong results in certain circumstances.


In xfs_seek_data(), every page past first dirty/unwritten page in the
unwritten extent will be reported as data.


in xfs_seek_data():
+		/*
+		 * Landed in an unwritten extent, try to find out the data
+		 * buffer offset from page cache firstly. If nothing was
+		 * found, treat it as a hole, and skip to check the next
+		 * extent, something just like above.
+		 */
+		if (map[0].br_state == XFS_EXT_UNWRITTEN) {
+			if (xfs_has_unwritten_buffer(inode, &map[0],
+						     PAGECACHE_TAG_DIRTY,
+						     &offset) ||
+			    xfs_has_unwritten_buffer(inode, &map[0],
+						     PAGECACHE_TAG_WRITEBACK,
+						     &offset)) {
+				offset = max_t(loff_t, seekoff, offset);
+				break;
+			}

Since the xfs_has_unwritten_buffer() returns the offset of the first
dirty/unwritten page (the first page in this example), the max_t()
comparison will say that every page after the first dirty page has
data.

		-----

xfs_seek_hole() can only find a hole if it precedes the first dirty
page.

in xfs_seek_hole():
+		/*
+		 * Landed in an unwritten extent, try to lookup the page
+		 * cache to find out if there is dirty data or not. If
+		 * nothing was found, treate it as a hole. If there has
+		 * dirty data and its offset starts past both the start
+		 * block of the map and the current seek offset, it should
+		 * be treated as hole too. Otherwise, go through the next
+		 * extent to fetch holes.
+		 */
+		if (map[0].br_state == XFS_EXT_UNWRITTEN) {
+			if (xfs_has_unwritten_buffer(inode, &map[0],
+						     PAGECACHE_TAG_DIRTY,
+						     &offset) ||
+			    xfs_has_unwritten_buffer(inode, &map[0],
+						     PAGECACHE_TAG_WRITEBACK,
+						     &offset)) {
+				if (offset > max_t(loff_t, seekoff,
+						   XFS_FSB_TO_B(mp,
+						   map[0].br_startoff))) {
+					offset = max_t(loff_t, seekoff,
+						       XFS_FSB_TO_B(mp,
+						       map[0].br_startoff));
+					break;
+				}
+			} else {
+				offset = max_t(loff_t, seekoff,
+					XFS_FSB_TO_B(mp, map[0].br_startoff));
+				break;
+			}

--Mark Tinguely.