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

[Dmitry Vyukov]

On Thu, Jan 14, 2021 at 2:00 PM Alexander Lobakin <alobakin@xxxxx> 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.