[PATCH v3 1/2] kfence: move the size check to the beginning of __kfence_alloc()

[Alexander Potapenko]

Check the allocation size before toggling kfence_allocation_gate.
This way allocations that can't be served by KFENCE will not result in
waiting for another CONFIG_KFENCE_SAMPLE_INTERVAL without allocating
anything.

Suggested-by: Marco Elver <elver@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
Cc: Marco Elver <elver@xxxxxxxxxx>
Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # 5.12+
Signed-off-by: Alexander Potapenko <glider@xxxxxxxxxx>
---
 mm/kfence/core.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/mm/kfence/core.c b/mm/kfence/core.c
index 4d21ac44d5d35..33bb20d91bf6a 100644
--- a/mm/kfence/core.c
+++ b/mm/kfence/core.c
@@ -733,6 +733,13 @@ void kfence_shutdown_cache(struct kmem_cache *s)
 
 void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
 {
+	/*
+	 * Perform size check before switching kfence_allocation_gate, so that
+	 * we don't disable KFENCE without making an allocation.
+	 */
+	if (size > PAGE_SIZE)
+		return NULL;
+
 	/*
 	 * allocation_gate only needs to become non-zero, so it doesn't make
 	 * sense to continue writing to it and pay the associated contention
@@ -757,9 +764,6 @@ void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
 	if (!READ_ONCE(kfence_enabled))
 		return NULL;
 
-	if (size > PAGE_SIZE)
-		return NULL;
-
 	return kfence_guarded_alloc(s, size, flags);
 }
 
-- 
2.32.0.93.g670b81a890-goog