linux-kernel - Re: [RFC PATCH net-next v5 2/2] net: add netmem to skb_frag

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Date: Thu, 11 Jan 2024 16:34:55 -0800
From: Mina Almasry <almasrymina@...gle.com>
To: Yunsheng Lin <linyunsheng@...wei.com>
Cc: linux-kernel@...r.kernel.org, netdev@...r.kernel.org, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Jason Gunthorpe <jgg@...dia.com>, 
	Christian König <christian.koenig@....com>, 
	Shakeel Butt <shakeelb@...gle.com>, Willem de Bruijn <willemdebruijn.kernel@...il.com>
Subject: Re: [RFC PATCH net-next v5 2/2] net: add netmem to skb_frag_t

On Thu, Jan 11, 2024 at 4:45 AM Yunsheng Lin <linyunsheng@...wei.com> wrote:
>
> On 2024/1/9 9:14, Mina Almasry wrote:
>
> ...
>
> >
> > +             if (WARN_ON_ONCE(!skb_frag_page(&skb_shinfo(skb)->frags[0]))) {
>
> I am really hate to bring it up again.
> If you are not willing to introduce a new helper,

I'm actually more than happy to add a new helper like:

static inline netmem_ref skb_frag_netmem();

For future callers to obtain frag->netmem to use the netmem_ref directly.

What I'm hung up on is really your follow up request:

"Is it possible to introduce something like skb_frag_netmem() for
netmem? so that we can keep most existing users of skb_frag_page()
unchanged and avoid adding additional checking overhead for existing
users."

With this patchseries, skb_frag_t no longer has a page pointer inside
of it, it only has a netmem_ref. The netmem_ref is currently always a
page, but in the future may not be a page. Can you clarify how we keep
skb_frag_page() unchanged and without checks? What do you expect
skb_frag_page() and its callers to do? We can not assume netmem_ref is
always a struct page. I'm happy to implement a change but I need to
understand it a bit better.

> do you care to use some
> existing API like skb_frag_address_safe()?
>

skb_frag_address_safe() checks that the page is mapped. In this case,
we are not checking if the frag page is mapped; we would like to make
sure that the skb_frag has a page inside of it in the first place.
Seems like a different check from skb_frag_address_safe().

In fact, skb_frag_address[_safe]() actually assume that the skb frag
is always a page currently, I think I need to squash this fix:

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index e59f76151628..bc8b107d0235 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -3533,7 +3533,9 @@ static inline void skb_frag_unref(struct sk_buff
*skb, int f)
  */
 static inline void *skb_frag_address(const skb_frag_t *frag)
 {
-       return page_address(skb_frag_page(frag)) + skb_frag_off(frag);
+       return skb_frag_page(frag) ?
+               page_address(skb_frag_page(frag)) + skb_frag_off(frag) :
+               NULL;
 }

 /**
@@ -3545,7 +3547,14 @@ static inline void *skb_frag_address(const
skb_frag_t *frag)
  */
 static inline void *skb_frag_address_safe(const skb_frag_t *frag)
 {
-       void *ptr = page_address(skb_frag_page(frag));
+       struct page *page;
+       void *ptr;
+
+       page = skb_frag_page(frag);
+       if (!page)
+               return NULL;
+
+       ptr = page_address(skb_frag_page(frag));
        if (unlikely(!ptr))
                return NULL;

> > +                     ret = -EINVAL;
> > +                     goto out;
> > +             }
> > +
> >               iov_iter_bvec(&msg.msg_iter, ITER_SOURCE,
> > -                           skb_shinfo(skb)->frags, skb_shinfo(skb)->nr_frags,
> > -                           msize);
> > +                           (const struct bio_vec *)skb_shinfo(skb)->frags,
> > +                           skb_shinfo(skb)->nr_frags, msize);
>
> I think we need to use some built-time checking to ensure some consistency
> between skb_frag_t and bio_vec.
>

I can add static_assert() that bio_vec->bv_len & bio_vec->bv_offset
are aligned with skb_frag_t->len & skb_frag_t->offset.

I can also maybe add a helper skb_frag_bvec() to do the cast instead
of doing it at the calling site. That may be a bit cleaner.

> >               iov_iter_advance(&msg.msg_iter, txm->frag_offset);
> >
> >               do {
> >

--
Thanks,
Mina