Re: [RFC 05/12] net: optimise page get/free for bvec zc

[Pavel Begunkov]

On 12/1/21 19:20, Jonathan Lemon wrote:
> On Tue, Nov 30, 2021 at 03:18:53PM +0000, Pavel Begunkov wrote:
> > get_page() in __zerocopy_sg_from_bvec() and matching put_page()s are
> > expensive. However, we can avoid it if the caller can guarantee that
> > pages stay alive until the corresponding ubuf_info is not released.
> > In particular, it targets io_uring with fixed buffers following the
> > described contract.
> >
> > Assuming that nobody yet uses bvec together with zerocopy, make all
> > calls with bvec iterators follow this model.
> >
> > Signed-off-by: Pavel Begunkov <asml.silence@xxxxxxxxx>
> > ---
[...]
> > diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> > index b23db60ea6f9..b7b087815539 100644
> > --- a/net/core/skbuff.c
> > +++ b/net/core/skbuff.c
> > @@ -666,10 +666,14 @@ static void skb_release_data(struct sk_buff *skb)
> >   			      &shinfo->dataref))
> >   		goto exit;
> >   
> > -	skb_zcopy_clear(skb, true);
> > +	if (!(shinfo->flags & SKBFL_MANAGED_FRAGS)) {
> > +		for (i = 0; i < shinfo->nr_frags; i++)
> > +			__skb_frag_unref(&shinfo->frags[i], skb->pp_recycle);
> > +	} else {
> > +		shinfo->flags &= ~SKBFL_MANAGED_FRAGS;
> > +	}
> >   
> > -	for (i = 0; i < shinfo->nr_frags; i++)
> > -		__skb_frag_unref(&shinfo->frags[i], skb->pp_recycle);
> > +	skb_zcopy_clear(skb, true);
>
> It would be better if all of this logic was moved into the callback
> under zcopy_clear.  Note that this path is called for both TX and RX.

Yeah, can be done and removes the extra if from non-zc path.
Thanks!

--
Pavel Begunkov