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

From: Dmitry Vyukov <dvyukov@google.com>
To: Alexander Lobakin <alobakin@pm.me>
Cc: Eric Dumazet <edumazet@google.com>,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	Edward Cree <ecree.xilinx@gmail.com>,
	Jonathan Lemon <jonathan.lemon@gmail.com>,
	Willem de Bruijn <willemb@google.com>,
	Miaohe Lin <linmiaohe@huawei.com>,
	Steffen Klassert <steffen.klassert@secunet.com>,
	Guillaume Nault <gnault@redhat.com>,
	Yadu Kishore <kyk.segfault@gmail.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	netdev <netdev@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 net-next 2/3] skbuff: (re)use NAPI skb cache on allocation path
Date: Thu, 14 Jan 2021 12:47:31 +0100	[thread overview]
Message-ID: <CACT4Y+adbmvvbzFnzRZzmpdTipg7ye53uR6OrnU9_K030sfzzA@mail.gmail.com> (raw)
In-Reply-To: <20210114114046.7272-1-alobakin@pm.me>

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.