On 2023/6/16 0:19, Jesper Dangaard Brouer wrote:

...

quoted

You have mentioned veth as the use-case. I know I acked adding page_pool
use-case to veth, for when we need to convert an SKB into an
xdp_buff/xdp-frame, but maybe it was the wrong hammer(?).
In this case in veth, the size is known at the page allocation time.
Thus, using the page_pool API is wasting memory.  We did this for
performance reasons, but we are not using PP for what is was intended
for.  We mostly use page_pool, because it an existing recycle return
path, and we were too lazy to add another alloc-type (see enum
xdp_mem_type).

Maybe you/we can extend veth to use this dynamic size API, to show us
that this is API is a better approach.  I will signup for benchmarking
this (and coordinating with CC Maryam as she came with use-case we
improved on).

Thanks, let's find out if page pool is the right hammer for the
veth XDP case.

Below is the change for veth using the new api in this patch.
Only compile test as I am not familiar enough with veth XDP and
testing environment for it.
Please try it if it is helpful.

diff --git a/drivers/net/veth.c b/drivers/net/veth.c
index 614f3e3efab0..8850394f1d29 100644
--- a/drivers/net/veth.c
+++ b/drivers/net/veth.c

@@ -736,7 +736,7 @@ static int veth_convert_skb_to_xdp_buff(struct veth_rq *rq,
         if (skb_shared(skb) || skb_head_is_locked(skb) ||
             skb_shinfo(skb)->nr_frags ||
             skb_headroom(skb) < XDP_PACKET_HEADROOM) {
-               u32 size, len, max_head_size, off;
+               u32 size, len, max_head_size, off, truesize, page_offset;
                 struct sk_buff *nskb;
                 struct page *page;
                 int i, head_off;

@@ -752,12 +752,15 @@ static int veth_convert_skb_to_xdp_buff(struct veth_rq *rq,
                 if (skb->len > PAGE_SIZE * MAX_SKB_FRAGS + max_head_size)
                         goto drop;

+               size = min_t(u32, skb->len, max_head_size);
+               truesize = size;
+
                 /* Allocate skb head */
-               page = page_pool_dev_alloc_pages(rq->page_pool);
+               page = page_pool_dev_alloc(rq->page_pool, &page_offset, &truesize);

                 if (!nskb) {
                         page_pool_put_full_page(rq->page_pool, page, true);
                         goto drop;

@@ -767,7 +770,6 @@ static int veth_convert_skb_to_xdp_buff(struct veth_rq *rq,
                 skb_copy_header(nskb, skb);
                 skb_mark_for_recycle(nskb);

-               size = min_t(u32, skb->len, max_head_size);
                 if (skb_copy_bits(skb, 0, nskb->data, size)) {
                         consume_skb(nskb);
                         goto drop;

@@ -782,14 +784,17 @@ static int veth_convert_skb_to_xdp_buff(struct veth_rq *rq,
                 len = skb->len - off;

                 for (i = 0; i < MAX_SKB_FRAGS && off < skb->len; i++) {
-                       page = page_pool_dev_alloc_pages(rq->page_pool);
+                       size = min_t(u32, len, PAGE_SIZE);
+                       truesize = size;
+
+                       page = page_pool_dev_alloc(rq->page_pool, &page_offset,
+                                                  &truesize);
                         if (!page) {
                                 consume_skb(nskb);
                                 goto drop;
                         }

-                       size = min_t(u32, len, PAGE_SIZE);
-                       skb_add_rx_frag(nskb, i, page, 0, size, PAGE_SIZE);
+                       skb_add_rx_frag(nskb, i, page, page_offset, size, truesize);