netdev - RE: [PATCH bpf-next 2/2] net: xdp: add xdp_update_skb_shared

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Message-ID: <60ec8dfeb42aa_50e1d20857@john-XPS-13-9370.notmuch>
Date:   Mon, 12 Jul 2021 11:46:22 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Lorenzo Bianconi <lorenzo@...nel.org>, bpf@...r.kernel.org
Cc:     netdev@...r.kernel.org, lorenzo.bianconi@...hat.com,
        davem@...emloft.net, kuba@...nel.org, ast@...nel.org,
        daniel@...earbox.net, alexander.duyck@...il.com, brouer@...hat.com,
        echaudro@...hat.com, magnus.karlsson@...el.com,
        tirthendu.sarkar@...el.com, toke@...hat.com
Subject: RE: [PATCH bpf-next 2/2] net: xdp: add xdp_update_skb_shared_info
 utility routine

Lorenzo Bianconi wrote:
> Introduce xdp_update_skb_shared_info routine to update frags array
> metadata from a given xdp_buffer/xdp_frame. We do not need to reset
> frags array since it is already initialized by the driver.
> Rely on xdp_update_skb_shared_info in mvneta driver.

Some more context here would really help. I had to jump into the mvneta
driver to see what is happening.

So as I read this we have a loop processing the descriptor in
mvneta_rx_swbm()

 mvneta_rx_swbm()
   while (rx_proc < budget && rx_proc < rx_todo) {
     if (rx_status & MVNETA_RXD_FIRST_DESC) ...
     else {
       mvneta_swbm_add_rx_fragment()
     }
     ..
     if (!rx_status & MVNETA_RXD_LAST_DESC)
         continue;
     ..
     if (xdp_prog)
       mvneta_run_xdp(...)
   }

roughly looking like above. First question, do you ever hit
!MVNETA_RXD_LAST_DESC today? I assume this is avoided by hardware
setup when XDP is enabled, otherwise _run_xdp() would be
broken correct? Next question, given last descriptor bit
logic whats the condition to hit the code added in this patch?
wouldn't we need more than 1 descriptor and then we would
skip the xdp_run... sorry lost me and its probably easier
to let you give the flow vs spending an hour trying to
track it down.

But, in theory as you handle a hardware discriptor you can build
up a set of pages using them to create a single skb rather than a
skb per descriptor. But don't we know if pfmemalloc should be
done while we are building the frag list? Can't se just set it
vs this for loop in xdp_update_skb_shared_info(),

> +	for (i = 0; i < nr_frags; i++) {
> +		struct page *page = skb_frag_page(&sinfo->frags[i]);
> +
> +		page = compound_head(page);
> +		if (page_is_pfmemalloc(page)) {
> +			skb->pfmemalloc = true;
> +			break;
> +		}
> +	}
> +}

...

> diff --git a/drivers/net/ethernet/marvell/mvneta.c b/drivers/net/ethernet/marvell/mvneta.c
> index 361bc4fbe20b..abf2e50880e0 100644
> --- a/drivers/net/ethernet/marvell/mvneta.c
> +++ b/drivers/net/ethernet/marvell/mvneta.c
> @@ -2294,18 +2294,29 @@ mvneta_swbm_add_rx_fragment(struct mvneta_port *pp,
>  	rx_desc->buf_phys_addr = 0;
>  
>  	if (data_len > 0 && xdp_sinfo->nr_frags < MAX_SKB_FRAGS) {
> -		skb_frag_t *frag = &xdp_sinfo->frags[xdp_sinfo->nr_frags++];
> +		skb_frag_t *frag = &xdp_sinfo->frags[xdp_sinfo->nr_frags];
>  
>  		skb_frag_off_set(frag, pp->rx_offset_correction);
>  		skb_frag_size_set(frag, data_len);
>  		__skb_frag_set_page(frag, page);
> +		/* We don't need to reset pp_recycle here. It's already set, so
> +		 * just mark fragments for recycling.
> +		 */
> +		page_pool_store_mem_info(page, rxq->page_pool);
> +
> +		/* first fragment */
> +		if (!xdp_sinfo->nr_frags)
> +			xdp_sinfo->gso_type = *size;

Would be nice to also change 'int size' -> 'unsigned int size' so the
types matched. Presumably you really can't have a negative size.

Also how about giving gso_type a better name. xdp_sinfo->size maybe?


> +		xdp_sinfo->nr_frags++;
>  
>  		/* last fragment */
>  		if (len == *size) {
>  			struct skb_shared_info *sinfo;
>  
>  			sinfo = xdp_get_shared_info_from_buff(xdp);
> +			sinfo->xdp_frags_tsize = xdp_sinfo->nr_frags * PAGE_SIZE;
>  			sinfo->nr_frags = xdp_sinfo->nr_frags;
> +			sinfo->gso_type = xdp_sinfo->gso_type;
>  			memcpy(sinfo->frags, xdp_sinfo->frags,
>  			       sinfo->nr_frags * sizeof(skb_frag_t));
>  		}

Thanks,
John