netdev - Re: [PATCH net-next 1/4] net: xdp: introduce bulking for xdp tx return path

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Date:   Wed, 28 Oct 2020 12:59:51 +0200
From:   Ilias Apalodimas <ilias.apalodimas@...aro.org>
To:     Lorenzo Bianconi <lorenzo@...nel.org>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org,
        lorenzo.bianconi@...hat.com, davem@...emloft.net, kuba@...nel.org,
        brouer@...hat.com
Subject: Re: [PATCH net-next 1/4] net: xdp: introduce bulking for xdp tx
 return path

On Wed, Oct 28, 2020 at 11:23:04AM +0100, Lorenzo Bianconi wrote:
> > Hi Lorenzo,
> 
> Hi Ilias,
> 
> thx for the review.
> 
> > 
> > On Tue, Oct 27, 2020 at 08:04:07PM +0100, Lorenzo Bianconi wrote:
> 
> [...]
> 
> > > +void xdp_return_frame_bulk(struct xdp_frame *xdpf,
> > > +			   struct xdp_frame_bulk *bq)
> > > +{
> > > +	struct xdp_mem_info *mem = &xdpf->mem;
> > > +	struct xdp_mem_allocator *xa, *nxa;
> > > +
> > > +	if (mem->type != MEM_TYPE_PAGE_POOL) {
> > > +		__xdp_return(xdpf->data, &xdpf->mem, false);
> > > +		return;
> > > +	}
> > > +
> > > +	rcu_read_lock();
> > > +
> > > +	xa = bq->xa;
> > > +	if (unlikely(!xa || mem->id != xa->mem.id)) {
> > 
> > Why is this marked as unlikely? The driver passes it as NULL. Should unlikely be
> > checked on both xa and the comparison?
> 
> xa is NULL only for the first xdp_frame in the burst while it is set for
> subsequent ones. Do you think it is better to remove it?

Ah correct, missed the general context of the driver this runs in.

> 
> > 
> > > +		nxa = rhashtable_lookup(mem_id_ht, &mem->id, mem_id_rht_params);
> > 
> > Is there a chance nxa can be NULL?
> 
> I do not think so since the page_pool is not destroyed while there are
> in-flight pages, right?

I think so but I am not 100% sure. I'll apply the patch and have a closer look

Cheers
/Ilias