Re: [PATCH bpf-next 2/2] net: xdp: add xdp_update_skb_shared_info utility routine

Lorenzo Bianconi wrote:

quoted

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.

Hi John,

ack, you are right. I will add more context next time. Sorry for the noise.

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.

I will point it out in the new commit log, but this is a preliminary patch for
xdp multi-buff support. In the current codebase xdp_update_skb_shared_info()
is run just when the NIC is not running in XDP mode (please note
mvneta_swbm_add_rx_fragment() is run even if xdp_prog is NULL).
When we add xdp multi-buff support, xdp_update_skb_shared_info() will run even
in XDP mode since we will remove the MTU constraint.

In the current codebsae the following condition can occur in non-XDP mode if
the packet is split on 3 or more descriptors (e.g. MTU 9000):

if (!(rx_status & MVNETA_RXD_LAST_DESC))
   continue;

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(),

I added pfmemalloc code in xdp_update_skb_shared_info() in order to reuse it
for the xdp_redirect use-case (e.g. whenever we redirect a xdp multi-buff
in a veth or in a cpumap). I have a pending patch where I am using
xdp_update_skb_shared_info in __xdp_build_skb_from_frame().

quoted

+	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;
+		}
+	}
+}

...

quoted

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.

ack

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

I did it in this way in order to avoid adding a union in skb_shared_info.
What about adding an inline helper to set/get it? e.g.

static inline u32 xdp_get_data_len(struct skb_shared_info *sinfo)
{
    return sinfo->gso_type;
}

static inline void xdp_set_data_len(struct skb_shared_info *sinfo, u32 datalen)
{
    sinfo->gso_type = datalen;
}

Regards,
Lorenzo

quoted

+		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