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

[Dmitry Vyukov]

On Fri, 17 Sept 2021 at 13:08, 'Marco Elver' via kasan-dev
<kasan-dev@xxxxxxxxxxxxxxxx> wrote:
>
> 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;

Do we expect this case to be so rare that we don't care?
Strictly speaking it's not no_capacity. So if I see large no_capacity
numbers, the first question I will have is: is it really no_capacity,
or some other case that we mixed together?



>         }
>
>         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
>
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