Messages in this thread | | | From | Dmitry Vyukov <> | Date | Thu, 14 Jan 2021 14:01:59 +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 2:00 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 > > Ah, I though of this too. But wouldn't there be a false-positive if > a poisoned skb hits kmem_cache_free_bulk(), not the allocation path? > We plan to use skb_cache for both reusing and bulk-freeing, and SLUB, > for example, might do writes into objects before freeing. > If it also should get unpoisoned before kmem_cache_free_bulk(), we'll > lose bulking as unpoisoning is performed per-object.
Yes, it needs to be unpoisoned before free. Unpoison one-by-one, free in bulk. Unpoisoningin is debug-only code anyway.
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