xfs: probe data buffer from page cache for unwritten extents

index 9f7ec15..a8821c1 100644
@@ -36,6 +36,7 @@
 
 #include <linux/dcache.h>
 #include <linux/falloc.h>
+#include <linux/pagevec.h>
 
 static const struct vm_operations_struct xfs_file_vm_ops;
 

@@ -966,6 +967,108 @@ xfs_vm_page_mkwrite(
 	return block_page_mkwrite(vma, vmf, xfs_get_blocks);
 }
 
+/*
+ * Probe the data buffer offset in page cache for unwritten extents.
+ * Iterate each page to find out if a buffer head state has BH_Unwritten
+ * or BH_Uptodate set.
+ */
+STATIC bool
+xfs_has_unwritten_buffer(
+	struct inode		*inode,
+	struct xfs_bmbt_irec	*map,
+	loff_t			*offset)
+{
+	struct xfs_inode	*ip = XFS_I(inode);
+	struct xfs_mount	*mp = ip->i_mount;
+	struct pagevec		pvec;
+	pgoff_t			index;
+	pgoff_t			end;
+	bool			found = false;
+
+	pagevec_init(&pvec, 0);
+
+	index = XFS_FSB_TO_B(mp, XFS_B_TO_FSBT(mp, *offset))
+			     >> PAGE_CACHE_SHIFT;
+	end = XFS_FSB_TO_B(mp, map->br_startoff + map->br_blockcount)
+			   >> PAGE_CACHE_SHIFT;
+	do {
+		unsigned int	i;
+		unsigned	nr_pages;
+		int		want = min_t(pgoff_t, end - index,
+					     (pgoff_t)PAGEVEC_SIZE - 1) + 1;
+		nr_pages = pagevec_lookup(&pvec, inode->i_mapping, index,
+					  want);
+		if (nr_pages == 0)
+			break;
+
+		for (i = 0; i < nr_pages; i++) {
+			struct page		*page = pvec.pages[i];
+			struct buffer_head	*bh;
+			struct buffer_head	*head;
+			xfs_fileoff_t		last;
+
+			/*
+			 * There is no need to check the following pages
+			 * if the current page offset is out of range.
+			 */
+			if (page->index > end)
+				goto out;
+
+			if (!trylock_page(page))
+				goto out;
+
+			if (!page_has_buffers(page)) {
+				unlock_page(page);
+				continue;
+			}
+
+			last = XFS_B_TO_FSBT(mp,
+					     page->index << PAGE_CACHE_SHIFT);
+			bh = head = page_buffers(page);
+			do {
+				/*
+				 * An extent in XFS_EXT_UNWRITTEN has disk
+				 * blocks already mapped to it, but no data
+				 * has been committed to them yet.  If it has
+				 * dirty data in the page cache it can be
+				 * identified by having BH_Unwritten set in
+				 * each buffers.  Also, the buffer head state
+				 * might be in BH_Uptodate too if the buffer
+				 * writeback procedure was fired, we need to
+				 * examine it as well.
+				 */
+				if ((buffer_unwritten(bh) ||
+				     buffer_uptodate(bh)) &&
+				     *offset <= XFS_FSB_TO_B(mp, last)) {
+					found = true;
+					*offset = XFS_FSB_TO_B(mp, last);
+					unlock_page(page);
+					goto out;
+				}
+				last++;
+			} while ((bh = bh->b_this_page) != head);
+			unlock_page(page);
+		}
+
+		/*
+		 * If the number of probed pages less than our desired,
+		 * there should no more pages mapped, search done.
+		 */
+		if (nr_pages < want)
+			break;
+
+		index = pvec.pages[i - 1]->index + 1;
+		pagevec_release(&pvec);
+	} while (index < end);
+
+out:
+	pagevec_release(&pvec);
+	if (!found)
+		*offset = 0;
+
+	return found;
+}
+
 STATIC loff_t
 xfs_seek_data(
 	struct file		*file,

@@ -975,8 +1078,6 @@ 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;

@@ -992,36 +1093,94 @@ 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;
 
-	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 {
-		if (nmap == 1) {
+		/* No extents at given offset, must be beyond EOF */
+		if (nmap == 0) {
 			error = ENXIO;
 			goto out_unlock;
 		}
 
-		offset = max_t(loff_t, start,
-			       XFS_FSB_TO_B(mp, map[1].br_startoff));
+		offset = start;
+		if (map[0].br_state == XFS_EXT_NORM &&
+		    !isnullstartblock(map[0].br_startblock))
+			break;
+		else {
+			/*
+			 * Landed in an unwritten extent, try to search
+			 * data buffer from page cache firstly.  Treat
+			 * it as a hole if nothing was found, and skip
+			 * to check the next extent.
+			 */
+			if (map[0].br_startblock == DELAYSTARTBLOCK ||
+			    map[0].br_state == XFS_EXT_UNWRITTEN) {
+				/* Probing page cache start from offset */
+				if (xfs_has_unwritten_buffer(inode, &map[0],
+							     &offset))
+					goto out;
+			}
+
+			/*
+			 * Found a hole in map[0] and nothing in map[1].
+			 * Probably means that we are reading after EOF.
+			 */
+			if (nmap == 1) {
+				error = ENXIO;
+				goto out_unlock;
+			}
+
+			/*
+			 * We have two mappings, and need to check map[1] to
+			 * see if there is data or not.
+			 */
+			offset = max_t(loff_t, start,
+				       XFS_FSB_TO_B(mp, map[1].br_startoff));
+
+			if (map[1].br_state == XFS_EXT_NORM &&
+			    !isnullstartblock(map[1].br_startblock))
+				goto out;
+			else {
+				/*
+				 * So map[1] is an unwritten extent as well,
+				 * try to search for data buffer in page cache.
+				 * We might find nothing if it is an allocated
+				 * and resvered extent.
+				 */
+				if (map[1].br_startblock == DELAYSTARTBLOCK ||
+				    map[1].br_state == XFS_EXT_UNWRITTEN) {
+					if (xfs_has_unwritten_buffer(inode,
+							&map[1], &offset))
+						goto out;
+				}
+			}
+		}
+
+		/*
+		 * Nothing was found, proceed to the next round of search if
+		 * reading offset not beyond EOF.
+		 */
+		fsbno = map[1].br_startoff + map[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;