FAILED: patch "[PATCH] stackdepot: stop preallocating after the final pool" failed to apply to 6.12-stable tree

[Sasha Levin <sashal@kernel.org>]

The patch below does not apply to the 6.12-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.12.y
git checkout FETCH_HEAD
git cherry-pick -x 8f56ea8e0706d92774603260779c7032f03ef3ac
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '20261107052548-failed-6.12-8f56ea8e0706@kernel.org' --subject-prefix 'PATCH 6.12.y' 'HEAD^..'

Thanks,
Sasha

------------------ original commit in Linus's tree ------------------

From 8f56ea8e0706d92774603260779c7032f03ef3ac Mon Sep 17 00:00:00 2001
From: Caleb Kan <ckan@cloudflare.com>
Date: Thu, 22 Oct 2026 09:33:30 +0100
Subject: [PATCH] stackdepot: stop preallocating after the final pool

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>
---
 lib/stackdepot.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/lib/stackdepot.c b/lib/stackdepot.c
index dd2717ff94bff..90c52f2e0d3f4 100644
--- a/lib/stackdepot.c
+++ b/lib/stackdepot.c
@@ -323,7 +323,7 @@ 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 < stack_max_pools)
+	if (pools_num + 1 < stack_max_pools)
 		WRITE_ONCE(new_pool, NULL);
 	else
 		WRITE_ONCE(new_pool, STACK_DEPOT_POISON);
-- 
2.53.0