kfence: count unexpectedly skipped allocations

[ Upstream commit 9a19aeb ]

Maintain a counter to count allocations that are skipped due to being
incompatible (oversized, incompatible gfp flags) or no capacity.

This is to compute the fraction of allocations that could not be
serviced by KFENCE, which we expect to be rare.

Link: https://lkml.kernel.org/r/20210923104803.2620285-2-elver@google.com
Signed-off-by: Marco Elver <elver@google.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Acked-by: Alexander Potapenko <glider@google.com>
Cc: Aleksandr Nogikh <nogikh@google.com>
Cc: Jann Horn <jannh@google.com>
Cc: Taras Madan <tarasmadan@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: melver

mm/kfence/core.c

@@ -113,6 +113,8 @@ enum kfence_counter_id {
 	KFENCE_COUNTER_FREES,
 	KFENCE_COUNTER_ZOMBIES,
 	KFENCE_COUNTER_BUGS,
+	KFENCE_COUNTER_SKIP_INCOMPAT,
+	KFENCE_COUNTER_SKIP_CAPACITY,
 	KFENCE_COUNTER_COUNT,
 };
 static atomic_long_t counters[KFENCE_COUNTER_COUNT];
@@ -122,6 +124,8 @@ static const char *const counter_names[] = {
 	[KFENCE_COUNTER_FREES]		= "total frees",
 	[KFENCE_COUNTER_ZOMBIES]	= "zombie allocations",
 	[KFENCE_COUNTER_BUGS]		= "total bugs",
+	[KFENCE_COUNTER_SKIP_INCOMPAT]	= "skipped allocations (incompatible)",
+	[KFENCE_COUNTER_SKIP_CAPACITY]	= "skipped allocations (capacity)",
 };
 static_assert(ARRAY_SIZE(counter_names) == KFENCE_COUNTER_COUNT);
 
@@ -272,8 +276,10 @@ static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t g
 		list_del_init(&meta->list);
 	}
 	raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
-	if (!meta)
+	if (!meta) {
+		atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_CAPACITY]);
 		return NULL;
+	}
 
 	if (unlikely(!raw_spin_trylock_irqsave(&meta->lock, flags))) {
 		/*
@@ -744,17 +750,21 @@ 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)
+	if (size > PAGE_SIZE) {
+		atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]);
 		return NULL;
+	}
 
 	/*
 	 * Skip allocations from non-default zones, including DMA. We cannot
 	 * guarantee that pages in the KFENCE pool will have the requested
 	 * properties (e.g. reside in DMAable memory).
 	 */
 	if ((flags & GFP_ZONEMASK) ||
-	    (s->flags & (SLAB_CACHE_DMA | SLAB_CACHE_DMA32)))
+	    (s->flags & (SLAB_CACHE_DMA | SLAB_CACHE_DMA32))) {
+		atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_INCOMPAT]);
 		return NULL;
+	}
 
 	if (atomic_inc_return(&kfence_allocation_gate) > 1)
 		return NULL;