[patch 206/262] kfence: count unexpectedly skipped allocations

Andrew Morton
2021-11-05T20:45:32Z

From: Marco Elver <elver@google.com>
Subject: kfence: count unexpectedly skipped allocations

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>
---

mm/kfence/core.c | 16 +++++++++++++---
1 file changed, 13 insertions(+), 3 deletions(-)

--- a/mm/kfence/core.c~kfence-count-unexpectedly-skipped-allocations
+++ a/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);

@@ -271,8 +275,10 @@ static void *kfence_guarded_alloc(struct
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))) {
/*
@@ -740,8 +746,10 @@ void *__kfence_alloc(struct kmem_cache *
* 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 +757,10 @@ void *__kfence_alloc(struct kmem_cache *
* 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
_