xfs: don't invalidate whole file on DAX read/write

When we do DAX IO, we try to invalidate the entire page cache held
on the file. This is incorrect as it will trash the entire mapping
tree that now tracks dirty state in exceptional entries in the radix
tree slots.

What we are trying to do is remove cached pages (e.g from reads
into holes) that sit in the radix tree over the range we are about
to write to. Hence we should just limit the invalidation to the
range we are about to overwrite.

Reported-by: Jan Kara <jack@suse.cz>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/xfs_file.c

@@ -741,9 +741,20 @@ xfs_file_dax_write(
 	 * page is inserted into the pagecache when we have to serve a write
 	 * fault on a hole.  It should never be dirtied and can simply be
 	 * dropped from the pagecache once we get real data for the page.
+	 *
+	 * XXX: This is racy against mmap, and there's nothing we can do about
+	 * it. dax_do_io() should really do this invalidation internally as
+	 * it will know if we've allocated over a holei for this specific IO and
+	 * if so it needs to update the mapping tree and invalidate existing
+	 * PTEs over the newly allocated range. Remove this invalidation when
+	 * dax_do_io() is fixed up.
 	 */
 	if (mapping->nrpages) {
-		ret = invalidate_inode_pages2(mapping);
+		loff_t end = iocb->ki_pos + iov_iter_count(from) - 1;
+
+		ret = invalidate_inode_pages2_range(mapping,
+						    iocb->ki_pos >> PAGE_SHIFT,
+						    end >> PAGE_SHIFT);
 		WARN_ON_ONCE(ret);
 	}