[PATCH 1/3] kfence: count unexpectedly skipped allocations

[Marco Elver <elver@xxxxxxxxxx>]

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.

Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
---
 mm/kfence/core.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/mm/kfence/core.c b/mm/kfence/core.c
index 7a97db8bc8e7..2755800f3e2a 100644
--- a/mm/kfence/core.c
+++ b/mm/kfence/core.c
@@ -112,6 +112,8 @@ 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];
@@ -121,6 +123,8 @@ 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);
 
@@ -272,7 +276,7 @@ static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t g
 	}
 	raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
 	if (!meta)
-		return NULL;
+		goto no_capacity;
 
 	if (unlikely(!raw_spin_trylock_irqsave(&meta->lock, flags))) {
 		/*
@@ -289,7 +293,7 @@ static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t g
 		list_add_tail(&meta->list, &kfence_freelist);
 		raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
 
-		return NULL;
+		goto no_capacity;
 	}
 
 	meta->addr = metadata_to_pageaddr(meta);
@@ -349,6 +353,10 @@ static void *kfence_guarded_alloc(struct kmem_cache *cache, size_t size, gfp_t g
 	atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCS]);
 
 	return addr;
+
+no_capacity:
+	atomic_long_inc(&counters[KFENCE_COUNTER_SKIP_CAPACITY]);
+	return NULL;
 }
 
 static void kfence_guarded_free(void *addr, struct kfence_metadata *meta, bool zombie)
@@ -740,8 +748,10 @@ 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
@@ -749,8 +759,10 @@ void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
 	 * 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;
+	}
 
 	/*
 	 * allocation_gate only needs to become non-zero, so it doesn't make
-- 
2.33.0.464.g1972c5931b-goog