[PATCH] mm: Fix readahead_page_batch for retry entries

["Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>]

Both btrfs and fuse have reported faults caused by seeing a retry
entry instead of the page they were looking for.  This was caused
by a missing check in the iterator.

Reported-by: David Sterba <dsterba@xxxxxxxx>
Reported-by: Wonhyuk Yang <vvghjk1234@xxxxxxxxx>
Fixes: 042124cc64c3 ("mm: add new readahead_control API")
Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index b921581a7dcf..2ea741c2bb16 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -751,6 +751,8 @@ static inline unsigned int __readahead_batch(struct readahead_control *rac,
 	xas_set(&xas, rac->_index);
 	rcu_read_lock();
 	xas_for_each(&xas, page, rac->_index + rac->_nr_pages - 1) {
+		if (xas_retry(&xas, page))
+			continue;
 		VM_BUG_ON_PAGE(!PageLocked(page), page);
 		VM_BUG_ON_PAGE(PageTail(page), page);
 		array[i++] = page;