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

[Marco Elver <elver@google.com>]

On Fri, 17 Sept 2021 at 14:58, Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Fri, 17 Sept 2021 at 13:08, 'Marco Elver' via kasan-dev
> <kasan-dev@googlegroups.com> 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@google.com>
> > ---
> >  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?

Hmm, true. I think we can just ignore counting this -- I'd expect some
bug-storm for this to become likely, at which point the system is in a
pretty bad state anyway (and we see bug counts increasing).

I'll remove this one.

>
>
> >         }
> >
> >         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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