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

[Dmitry Vyukov <dvyukov@google.com>]

On Thu, Jan 14, 2021 at 1:50 PM Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Thu, Jan 14, 2021 at 1:44 PM Alexander Lobakin <alobakin@pm.me> wrote:
> >
> > From: Dmitry Vyukov <dvyukov@google.com>
> > Date: Thu, 14 Jan 2021 12:47:31 +0100
> >
> > > On Thu, Jan 14, 2021 at 12:41 PM Alexander Lobakin <alobakin@pm.me> wrote:
> > >>
> > >> From: Eric Dumazet <edumazet@google.com>
> > >> Date: Wed, 13 Jan 2021 15:36:05 +0100
> > >>
> > >>> On Wed, Jan 13, 2021 at 2:37 PM Alexander Lobakin <alobakin@pm.me> 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

Variable-size poisoning/unpoisoning would be needed for the skb data itself:
https://bugzilla.kernel.org/show_bug.cgi?id=199055