vfs: Introduce filemap_fdatawait_range

This simple helper saves some filesystems conversion from byte offset
to page numbers and also makes the fdata* interface more complete.
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NJan Kara <jack@suse.cz>

include/linux/fs.h

@@ -2086,6 +2086,8 @@ extern int write_inode_now(struct inode *, int);
 extern int filemap_fdatawrite(struct address_space *);
 extern int filemap_flush(struct address_space *);
 extern int filemap_fdatawait(struct address_space *);
+extern int filemap_fdatawait_range(struct address_space *, loff_t lstart,
+				   loff_t lend);
 extern int filemap_write_and_wait(struct address_space *mapping);
 extern int filemap_write_and_wait_range(struct address_space *mapping,
 				        loff_t lstart, loff_t lend);

mm/filemap.c

@@ -306,6 +306,26 @@ int wait_on_page_writeback_range(struct address_space *mapping,
 	return ret;
 }
 
+/**
+ * filemap_fdatawait_range - wait for all under-writeback pages to complete in a given range
+ * @mapping: address space structure to wait for
+ * @start:	offset in bytes where the range starts
+ * @end:	offset in bytes where the range ends (inclusive)
+ *
+ * Walk the list of under-writeback pages of the given address space
+ * in the given range and wait for all of them.
+ *
+ * This is just a simple wrapper so that callers don't have to convert offsets
+ * to page indexes themselves
+ */
+int filemap_fdatawait_range(struct address_space *mapping, loff_t start,
+			    loff_t end)
+{
+	return wait_on_page_writeback_range(mapping, start >> PAGE_CACHE_SHIFT,
+					    end >> PAGE_CACHE_SHIFT);
+}
+EXPORT_SYMBOL(filemap_fdatawait_range);
+
 /**
  * sync_page_range - write and wait on all pages in the passed range
  * @inode:	target inode