[PATCH v1] mm/damon/pa: initialize 'folio' to NULL to prevent use of uninitialized value

[Aaron Yang]

In damon_pa_mark_accessed_or_deactivate(), the local variable 'folio'
is declared but not initialized. If the region [r->ar.start, r->ar.end)
is empty or invalid such that the while-loop body is never entered,
'folio' retains an indeterminate (garbage) value. The function then
unconditionally assigns this uninitialized pointer to s->last_applied
(line 239), resulting in undefined behavior. Subsequent dereference or
folio_put() on s->last_applied may cause crashes or memory corruption.

Although DAMON regions are typically non-empty, zero-length regions
can arise during region merging/splitting or due to address unit
alignment — making this path reachable in practice.

Fix by initializing 'folio' to NULL. Assigning NULL to s->last_applied
is safe and semantically correct: it cleanly indicates "no folio was
processed in this invocation", and callers are expected to check for
NULL before use (as per common kernel practice).

No functional change for non-empty regions; only hardens error/edge
case handling.

Signed-off-by: Aaron Yang <yangqixiao@xxxxxxxxxx>
---
 mm/damon/paddr.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/damon/paddr.c b/mm/damon/paddr.c
index 07a8aead439e..32d8024d130e 100644
--- a/mm/damon/paddr.c
+++ b/mm/damon/paddr.c
@@ -212,7 +212,7 @@ static inline unsigned long damon_pa_mark_accessed_or_deactivate(
 		unsigned long *sz_filter_passed)
 {
 	phys_addr_t addr, applied = 0;
-	struct folio *folio;
+	struct folio *folio = NULL;
 
 	addr = damon_pa_phys_addr(r->ar.start, addr_unit);
 	while (addr < damon_pa_phys_addr(r->ar.end, addr_unit)) {
-- 
2.47.3