From: Dmitry Vyukov <dvyu...@google.com> Date: Thu, 14 Jan 2021 13:50:25 +0100
> On Thu, Jan 14, 2021 at 1:44 PM Alexander Lobakin <aloba...@pm.me> wrote: >> >> From: Dmitry Vyukov <dvyu...@google.com> >> Date: Thu, 14 Jan 2021 12:47:31 +0100 >> >>> On Thu, Jan 14, 2021 at 12:41 PM Alexander Lobakin <aloba...@pm.me> wrote: >>>> >>>> From: Eric Dumazet <eduma...@google.com> >>>> Date: Wed, 13 Jan 2021 15:36:05 +0100 >>>> >>>>> On Wed, Jan 13, 2021 at 2:37 PM Alexander Lobakin <aloba...@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. Al