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

[Jonathan Lemon <jonathan.lemon@gmail.com>]

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@gmail.com>
> ---
>  include/linux/skbuff.h | 10 +++++++++-
>  net/core/datagram.c    |  9 +++++++--
>  net/core/skbuff.c      | 16 +++++++++++++---
>  net/ipv4/ip_output.c   |  4 ++++
>  4 files changed, 33 insertions(+), 6 deletions(-)
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 750b7518d6e2..ebb12a7d386d 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -461,6 +461,11 @@ enum {
>  	SKBFL_PURE_ZEROCOPY = BIT(2),
>  
>  	SKBFL_DONT_ORPHAN = BIT(3),
> +
> +	/* page references are managed by the ubuf_info, so it's safe to
> +	 * use frags only up until ubuf_info is released
> +	 */
> +	SKBFL_MANAGED_FRAGS = BIT(4),
>  };
>  
>  #define SKBFL_ZEROCOPY_FRAG	(SKBFL_ZEROCOPY_ENABLE | SKBFL_SHARED_FRAG)
> @@ -3154,7 +3159,10 @@ static inline void __skb_frag_unref(skb_frag_t *frag, bool recycle)
>   */
>  static inline void skb_frag_unref(struct sk_buff *skb, int f)
>  {
> -	__skb_frag_unref(&skb_shinfo(skb)->frags[f], skb->pp_recycle);
> +	struct skb_shared_info *shinfo = skb_shinfo(skb);
> +
> +	if (!(shinfo->flags & SKBFL_MANAGED_FRAGS))
> +		__skb_frag_unref(&shinfo->frags[f], skb->pp_recycle);
>  }
>  
>  /**
> diff --git a/net/core/datagram.c b/net/core/datagram.c
> index e00f7e0a7a0a..5cf0672039d6 100644
> --- a/net/core/datagram.c
> +++ b/net/core/datagram.c
> @@ -642,7 +642,6 @@ static int __zerocopy_sg_from_bvec(struct sock *sk, struct sk_buff *skb,
>  		v = mp_bvec_iter_bvec(from->bvec, bi);
>  		copied += v.bv_len;
>  		truesize += PAGE_ALIGN(v.bv_len + v.bv_offset);
> -		get_page(v.bv_page);
>  		skb_fill_page_desc(skb, frag++, v.bv_page, v.bv_offset, v.bv_len);
>  		bvec_iter_advance_single(from->bvec, &bi, v.bv_len);
>  	}
> @@ -671,9 +670,15 @@ int __zerocopy_sg_from_iter(struct sock *sk, struct sk_buff *skb,
>  			    struct iov_iter *from, size_t length)
>  {
>  	int frag = skb_shinfo(skb)->nr_frags;
> +	bool managed = skb_shinfo(skb)->flags & SKBFL_MANAGED_FRAGS;
>  
> -	if (iov_iter_is_bvec(from))
> +	if (iov_iter_is_bvec(from) && (managed || frag == 0)) {
> +		skb_shinfo(skb)->flags |= SKBFL_MANAGED_FRAGS;
>  		return __zerocopy_sg_from_bvec(sk, skb, from, length);
> +	}
> +
> +	if (managed)
> +		return -EFAULT;
>  
>  	while (length && iov_iter_count(from)) {
>  		struct page *pages[MAX_SKB_FRAGS];
> 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.
-- 
Jonathan