[PATCH] mm: fix dereference a null pointer in migrate[_huge]_page_move_mapping()

[liqiong]

Upstream has no this bug.

The two functions look up a slot and dereference the pointer,
If the pointer is null, the kernel would crash and dump.

The 'numad' service calls 'migrate_pages' periodically. If some slots
being replaced (Cache Eviction), the radix_tree_lookup_slot() returns
a null pointer, then, kernel crash.

"numad":  crash> bt
[exception RIP: migrate_page_move_mapping+337]

Introduct a pointer checking to avoid dereference a null pointer.

Cc: <stable@xxxxxxxxxxxxxxx> # v4.19-rc8
Signed-off-by: liqiong <liqiong@xxxxxxxxxxxx>
---
 mm/migrate.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/mm/migrate.c b/mm/migrate.c
index 84381b55b2bd..1ff95c259511 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -474,6 +474,10 @@ int migrate_page_move_mapping(struct address_space *mapping,
 
 	pslot = radix_tree_lookup_slot(&mapping->i_pages,
  					page_index(page));
+	if (pslot == NULL) {
+		xa_unlock_irq(&mapping->i_pages);
+		return -EAGAIN;
+	}
 
 	expected_count += hpage_nr_pages(page) + page_has_private(page);
 	if (page_count(page) != expected_count ||
@@ -592,6 +596,10 @@ int migrate_huge_page_move_mapping(struct address_space *mapping,
 	xa_lock_irq(&mapping->i_pages);
 
 	pslot = radix_tree_lookup_slot(&mapping->i_pages, page_index(page));
+	if (pslot == NULL) {
+		xa_unlock_irq(&mapping->i_pages);
+		return -EAGAIN;
+	}
 
 	expected_count = 2 + page_has_private(page);
 	if (page_count(page) != expected_count ||
-- 
2.25.1