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

Lorenzo Bianconi wrote:

quoted

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.

quoted

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, as is there is no caller of xdp_update_skb_shared_info() so
I think we should move the these two patches into the series with
the multibuf support.

quoted

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().

OK, but it adds an extra for loop and the related overhead. Can
we avoid this overhead and just set it from where we first
know we have a compound page. Or carry some bit through and
do a simpler check,

 if (pfmemalloc_needed) skb->pfmemalloc = true;

I guess in the case here its building the skb so performance is maybe
not as critical, but if it gets used in the redirect case then we
shouldn't be doing unnecessary for loops.

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+	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

quoted

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.

What was wrong with the union?

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