Re: [PATCH v2 net-next 2/3] skbuff: (re)use NAPI skb cache on allocation path

[Dmitry Vyukov]

On Thu, Jan 14, 2021 at 1:44 PM Alexander Lobakin <alobakin@xxxxx> wrote:
>
> From: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> Date: Thu, 14 Jan 2021 12:47:31 +0100
>
> > On Thu, Jan 14, 2021 at 12:41 PM Alexander Lobakin <alobakin@xxxxx> wrote:
> >>
> >> From: Eric Dumazet <edumazet@xxxxxxxxxx>
> >> Date: Wed, 13 Jan 2021 15:36:05 +0100
> >>
> >>> On Wed, Jan 13, 2021 at 2:37 PM Alexander Lobakin <alobakin@xxxxx> wrote:
> >>>>
> >>>> Instead of calling kmem_cache_alloc() every time when building a NAPI
> >>>> skb, (re)use skbuff_heads from napi_alloc_cache.skb_cache. Previously
> >>>> this cache was only used for bulk-freeing skbuff_heads consumed via
> >>>> napi_consume_skb() or __kfree_skb_defer().
> >>>>
> >>>> Typical path is:
> >>>>  - skb is queued for freeing from driver or stack, its skbuff_head
> >>>>    goes into the cache instead of immediate freeing;
> >>>>  - driver or stack requests NAPI skb allocation, an skbuff_head is
> >>>>    taken from the cache instead of allocation.
> >>>>
> >>>> Corner cases:
> >>>>  - if it's empty on skb allocation, bulk-allocate the first half;
> >>>>  - if it's full on skb consuming, bulk-wipe the second half.
> >>>>
> >>>> Also try to balance its size after completing network softirqs
> >>>> (__kfree_skb_flush()).
> >>>
> >>> I do not see the point of doing this rebalance (especially if we do not change
> >>> its name describing its purpose more accurately).
> >>>
> >>> For moderate load, we will have a reduced bulk size (typically one or two).
> >>> Number of skbs in the cache is in [0, 64[ , there is really no risk of
> >>> letting skbs there for a long period of time.
> >>> (32 * sizeof(sk_buff) = 8192)
> >>> I would personally get rid of this function completely.
> >>
> >> When I had a cache of 128 entries, I had worse results without this
> >> function. But seems like I forgot to retest when I switched to the
> >> original size of 64.
> >> I also thought about removing this function entirely, will test.
> >>
> >>> Also it seems you missed my KASAN support request ?
> >>  I guess this is a matter of using kasan_unpoison_range(), we can ask for help.
> >>
> >> I saw your request, but don't see a reason for doing this.
> >> We are not caching already freed skbuff_heads. They don't get
> >> kmem_cache_freed before getting into local cache. KASAN poisons
> >> them no earlier than at kmem_cache_free() (or did I miss someting?).
> >> heads being cached just get rid of all references and at the moment
> >> of dropping to the cache they are pretty the same as if they were
> >> allocated.
> >
> > KASAN should not report false positives in this case.
> > But I think Eric meant preventing false negatives. If we kmalloc 17
> > bytes, KASAN will detect out-of-bounds accesses beyond these 17 bytes.
> > But we put that data into 128-byte blocks, KASAN will miss
> > out-of-bounds accesses beyond 17 bytes up to 128 bytes.
> > The same holds for "logical" use-after-frees when object is free, but
> > not freed into slab.
> >
> > An important custom cache should use annotations like
> > kasan_poison_object_data/kasan_unpoison_range.
>
> As I understand, I should
> kasan_poison_object_data(skbuff_head_cache, skb) and then
> kasan_unpoison_range(skb, sizeof(*skb)) when putting it into the
> cache?

I think it's the other way around. It should be _un_poisoned when used.
If it's fixed size, then unpoison_object_data should be a better fit:
https://elixir.bootlin.com/linux/v5.11-rc3/source/mm/kasan/common.c#L253