Messages in this thread | | | From | Dmitry Vyukov <> | Date | Thu, 14 Jan 2021 13:50:25 +0100 | Subject | Re: [PATCH v2 net-next 2/3] skbuff: (re)use NAPI skb cache on allocation path |
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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
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