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

[Alexander Lobakin <alobakin@pm.me>]

From: Dmitry Vyukov <dvyukov@google.com>
Date: Thu, 14 Jan 2021 13:51:44 +0100

> 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

This cache is for skbuff_heads only, not for the entire skbs. All
linear data and frags gets freed before head hits the cache.
The cache will store skbuff_heads as if they were freshly allocated
by kmem_cache_alloc().

Al