Re: [net PATCH] skb: Do mix page pool and page referenced frags in GRO

[Eric Dumazet]

On Sat, Jan 28, 2023 at 6:26 AM Jakub Kicinski <kuba@xxxxxxxxxx> wrote:
>
> On Sat, 28 Jan 2023 10:37:47 +0800 Yunsheng Lin wrote:
> > If we are not allowing gro for the above case, setting NAPI_GRO_CB(p)->flush
> > to 1 in gro_list_prepare() seems to be making more sense so that the above
> > case has the same handling as skb_has_frag_list() handling?
> > https://elixir.bootlin.com/linux/v6.2-rc4/source/net/core/gro.c#L503
> >
> > As it seems to avoid some unnecessary operation according to comment
> > in tcp4_gro_receive():
> > https://elixir.bootlin.com/linux/v6.2-rc4/source/net/ipv4/tcp_offload.c#L322
>
> The frag_list case can be determined with just the input skb.
> For pp_recycle we need to compare input skb's pp_recycle with
> the pp_recycle of the skb already held by GRO.
>
> I'll hold off with applying a bit longer tho, in case Eric
> wants to chime in with an ack or opinion.

We can say that we are adding in the fast path an expensive check
about an unlikely condition.

GRO is by far the most expensive component in our stack.

I would at least make the extra checks conditional to CONFIG_PAGE_POOL=y
Ideally all accesses to skb->pp_recycle should be done via a helper [1]

I hope that we will switch later to a per-page marker, instead of a per-skb one.

( a la https://www.spinics.net/lists/netdev/msg874099.html )

[1] Problem is that CONFIG_PAGE_POOL=y is now forced because of
net/bpf/test_run.c

So... testing skb->pp_recycle seems needed for the time being

Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>

My tentative patch was something like:

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 4c8492401a101f1d6d43079fc70962210389763c..a53b176738b10f3b69b38c487e0c280f44990b6f
100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -918,8 +918,10 @@ struct sk_buff {
                                fclone:2,
                                peeked:1,
                                head_frag:1,
-                               pfmemalloc:1,
-                               pp_recycle:1; /* page_pool recycle indicator */
+#ifdef CONFIG_PAGE_POOL
+                               pp_recycle:1, /* page_pool recycle indicator */
+#endif
+                               pfmemalloc:1;
 #ifdef CONFIG_SKB_EXTENSIONS
        __u8                    active_extensions;
 #endif
@@ -3388,6 +3390,15 @@ static inline void __skb_frag_unref(skb_frag_t
*frag, bool recycle)
        put_page(page);
 }

+static inline bool skb_pp_recycle(const struct sk_buff *skb)
+{
+#ifdef CONFIG_PAGE_POOL
+       return skb->pp_recycle;
+#else
+       return false;
+#endif
+}
+
 /**
  * skb_frag_unref - release a reference on a paged fragment of an skb.
  * @skb: the buffer
@@ -3400,7 +3411,7 @@ static inline void skb_frag_unref(struct sk_buff
*skb, int f)
        struct skb_shared_info *shinfo = skb_shinfo(skb);

        if (!skb_zcopy_managed(skb))
-               __skb_frag_unref(&shinfo->frags[f], skb->pp_recycle);
+               __skb_frag_unref(&shinfo->frags[f], skb_pp_recycle(skb));
 }

 /**
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 180df58e85c72eaa16f5cb56b56d181a379b8921..7a2783a2c9608eec728a0adacea4619ab1c62791
100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -801,19 +801,13 @@ static void skb_clone_fraglist(struct sk_buff *skb)
                skb_get(list);
 }

-static bool skb_pp_recycle(struct sk_buff *skb, void *data)
-{
-       if (!IS_ENABLED(CONFIG_PAGE_POOL) || !skb->pp_recycle)
-               return false;
-       return page_pool_return_skb_page(virt_to_page(data));
-}
-
 static void skb_free_head(struct sk_buff *skb)
 {
        unsigned char *head = skb->head;

        if (skb->head_frag) {
-               if (skb_pp_recycle(skb, head))
+               if (skb_pp_recycle(skb) &&
+                   page_pool_return_skb_page(virt_to_page(head)))
                        return;
                skb_free_frag(head);
        } else {
@@ -840,7 +834,7 @@ static void skb_release_data(struct sk_buff *skb,
enum skb_drop_reason reason)
        }

        for (i = 0; i < shinfo->nr_frags; i++)
-               __skb_frag_unref(&shinfo->frags[i], skb->pp_recycle);
+               __skb_frag_unref(&shinfo->frags[i], skb_pp_recycle(skb));

 free_head:
        if (shinfo->frag_list)
@@ -857,7 +851,10 @@ static void skb_release_data(struct sk_buff *skb,
enum skb_drop_reason reason)
         * Eventually the last SKB will have the recycling bit set and it's
         * dataref set to 0, which will trigger the recycling
         */
+#ifdef CONFIG_PAGE_POOL
        skb->pp_recycle = 0;
+#endif
+       return;
 }

 /*
@@ -1292,7 +1289,9 @@ static struct sk_buff *__skb_clone(struct
sk_buff *n, struct sk_buff *skb)
        n->nohdr = 0;
        n->peeked = 0;
        C(pfmemalloc);
+#ifdef CONFIG_PAGE_POOL
        C(pp_recycle);
+#endif
        n->destructor = NULL;
        C(tail);
        C(end);
@@ -3859,7 +3858,7 @@ int skb_shift(struct sk_buff *tgt, struct
sk_buff *skb, int shiftlen)
                fragto = &skb_shinfo(tgt)->frags[merge];

                skb_frag_size_add(fragto, skb_frag_size(fragfrom));
-               __skb_frag_unref(fragfrom, skb->pp_recycle);
+               __skb_frag_unref(fragfrom, skb_pp_recycle(skb));
        }

        /* Reposition in the original skb */
@@ -5529,7 +5528,7 @@ bool skb_try_coalesce(struct sk_buff *to, struct
sk_buff *from,
         * references for cloned SKBs at the moment that would result in
         * inconsistent reference counts.
         */
-       if (to->pp_recycle != (from->pp_recycle && !skb_cloned(from)))
+       if (skb_pp_recycle(to) != (skb_pp_recycle(from) && !skb_cloned(from)))
                return false;

        if (len <= skb_tailroom(to)) {