Re: [PATCH v2 bpf] xsk: fix immature cq descriptor production

[Stanislav Fomichev <stfomichev@gmail.com>]

On 07/08, Alexander Lobakin wrote:
> From: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
> Date: Tue, 8 Jul 2025 16:14:39 +0200
> 
> > On Mon, Jul 07, 2025 at 11:40:48AM -0700, Stanislav Fomichev wrote:
> >> On 07/07, Alexander Lobakin wrote:
> 
> [...]
> 
> >>> BTW isn't num_descs from that new structure would be the same as
> >>> shinfo->nr_frags + 1 (or just nr_frags for xsk_build_skb_zerocopy())?
> >>
> >> So you're saying we don't need to store it? Agreed. But storing the rest
> >> in cb still might be problematic with kconfig-configurable MAX_SKB_FRAGS?
> 
> For sure skb->cb is too small for 17+ u64s.
> 
> > 
> > Hi Stan & Olek,
> > 
> > no, as said in v1 drivers might linearize the skb and all frags will be
> > lost. This storage is needed unfortunately.
> 
> Aaah sorry. In this case yeah, you need this separate frag count.
> 
> > 
> >>
> >>>> Can we pre-allocate an array of xsk_addrs during xsk_bind (the number of
> >>>> xsk_addrs is bound by the tx ring size)? Then we can remove the alloc on tx
> >>>> and replace it with some code to manage that pool of xsk_addrs..
> > 
> > That would be pool-bound which makes it a shared resource so I believe
> > that we would repeat the problem being fixed here ;)
> 
> Except the system Page Pool idea right below maybe :>
 
 It doesn't have to be a shared resource, the pool (in whatever form) can be
 per xsk. (unless I'm missing something)

> >>> Nice idea BTW.
> >>>
> >>> We could even use system per-cpu Page Pools to allocate these structs*
> >>> :D It wouldn't waste 1 page per one struct as PP is frag-aware and has
> >>> API for allocating only a small frag.
> >>>
> >>> Headroom stuff was also ok to me: we either way allocate a new skb, so
> >>> we could allocate it with a bit bigger headroom and put that table there
> >>> being sure that nobody will overwrite it (some drivers insert special
> >>> headers or descriptors in front of the actual skb->data).
> > 
> > headroom approach was causing one of bpf selftests to fail, but I didn't
> > check in-depth the reason. I didn't really like the check in destructor if
> > addr array was corrupted in v1 and I came up with v2 which seems to me a
> > cleaner fix.
> > 
> >>>
> >>> [*] Offtop: we could also use system PP to allocate skbs in
> >>> xsk_build_skb() just like it's done in xdp_build_skb_from_zc() +
> >>> xdp_copy_frags_from_zc() -- no way to avoid memcpy(), but the payload
> >>> buffers would be recycled then.
> >>
> >> Or maybe kmem_cache_alloc_node with a custom cache is good enough?
> >> Headroom also feels ok if we store the whole xsk_addrs struct in it.
> > 
> > Yep both of these approaches was something I considered, but keep in mind
> > it's a bugfix so I didn't want to go with something flashy. I have not
> > observed big performance impact but I checked only MAX_SKB_FRAGS being set
> > to standard value.
> > 
> > Would you guys be ok if I do the follow-up with possible optimization
> > after my vacation which would be a -next candidate?
> 
> As a fix, it's totally fine for me to go in the current form, sure.

+1