[PATCH 6.12.y] stackdepot: stop preallocating after the final pool

[Sasha Levin <sashal@kernel.org>]

From: Caleb Kan <ckan@cloudflare.com>

[ Upstream commit 8f56ea8e0706d92774603260779c7032f03ef3ac ]

depot_init_pool() decides whether another pool is needed before it
increments pools_num. When it registers the last allowed pool, pools_num
is still one below stack_max_pools, so the existing comparison clears
new_pool.

With new_pool cleared, stack_depot_save_flags() can allocate another
order-2 pool after a lookup miss. depot_keep_new_pool() retains its pointer
in the global new_pool, so the allocation is not lost. However, pools_num
has already reached stack_max_pools, and depot_init_pool() rejects every
attempt to register the pool. It remains allocated and unusable until
reboot. The non-NULL pointer prevents later saves from allocating more
spare pools.

Account for the pool being registered in the limit check so registering
the final pool installs STACK_DEPOT_POISON and prevents the extra
allocation.

Link: https://lore.kernel.org/20261022083330.3159793-1-matt@readmodwrite.com
Fixes: 31639fd6cebd ("stackdepot: use variable size records for non-evictable entries")
Signed-off-by: Caleb Kan <ckan@cloudflare.com>
Signed-off-by: Matt Fleming <mfleming@cloudflare.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Cc: Caleb Kan <calebkan1106@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: <stable@vger.kernel.org>
[ sashal: Used the existing DEPOT_MAX_POOLS constant instead of upstream's stack_max_pools variable. ]
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 lib/stackdepot.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/stackdepot.c b/lib/stackdepot.c
index 245d5b4166999..1f1d63d21f26b 100644
--- a/lib/stackdepot.c
+++ b/lib/stackdepot.c
@@ -273,7 +273,7 @@ static bool depot_init_pool(void **prealloc)
 	 * NULL; do not reset to NULL if we have reached the maximum number of
 	 * pools.
 	 */
-	if (pools_num < DEPOT_MAX_POOLS)
+	if (pools_num + 1 < DEPOT_MAX_POOLS)
 		WRITE_ONCE(new_pool, NULL);
 	else
 		WRITE_ONCE(new_pool, STACK_DEPOT_POISON);
-- 
2.53.0