[PATCH 4.19 134/142] dax: Check page->mapping isnt NULL

[Greg Kroah-Hartman]

4.19-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Matthew Wilcox <willy@xxxxxxxxxxxxx>

commit c93db7bb6ef3251e0ea48ade311d3e9942748e1c upstream.

If we race with inode destroy, it's possible for page->mapping to be
NULL before we even enter this routine, as well as after having slept
waiting for the dax entry to become unlocked.

Fixes: c2a7d2a11552 ("filesystem-dax: Introduce dax_lock_mapping_entry()")
Cc: <stable@xxxxxxxxxxxxxxx>
Reported-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Reviewed-by: Johannes Thumshirn <jthumshirn@xxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 fs/dax.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/fs/dax.c
+++ b/fs/dax.c
@@ -423,7 +423,7 @@ bool dax_lock_mapping_entry(struct page
 	for (;;) {
 		mapping = READ_ONCE(page->mapping);
 
-		if (!dax_mapping(mapping))
+		if (!mapping || !dax_mapping(mapping))
 			break;
 
 		/*