[RFC] mm/huge_memory: prevent potential NULL pointer dereference

[Antonio Quartulli <antonio@mandelbit.com>]

I just found this issue in the last linux-next Coverity report and it
caught my attention.
I am not familiar with this code, therefore I am sending this patch
as RFC because I am not 100% sure whether this is a false positive or
not.
However, it seems potentially legit to me:

In __folio_split(), when looping over folios we dereference
`mapping` before ensuring it is non-NULL.

Following code in the loop body performs such check, thus
suggesting that `mapping` may be NULL and accessing it
without any check may be dangerous.

Add NULL check before passing it to shmem_mapping().

Cc: Zi Yan <ziy@nvidia.com>
Fixes: 00527733d0dc ("mm/huge_memory: add two new (not yet used) functions for folio_split()")
Addresses-Coverity-ID: 1647614 ("FORWARD_NULL")
Signed-off-by: Antonio Quartulli <antonio@mandelbit.com>
---
 mm/huge_memory.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 389620c65a5f..d649026db95a 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -3802,7 +3802,7 @@ static int __folio_split(struct folio *folio, unsigned int new_order,
 
 			/* Some pages can be beyond EOF: drop them from cache */
 			if (new_folio->index >= end) {
-				if (shmem_mapping(mapping))
+				if (mapping && shmem_mapping(mapping))
 					nr_shmem_dropped += folio_nr_pages(new_folio);
 				else if (folio_test_clear_dirty(new_folio))
 					folio_account_cleaned(
-- 
2.49.1