netdev - Re: [PATCH v1 net-next 01/15] iov_iter: Skip copy in memcpy_to

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Date:   Mon, 7 Dec 2020 17:39:40 -0700
From:   David Ahern <dsahern@...il.com>
To:     Boris Pismenny <borisp@...lanox.com>, kuba@...nel.org,
        davem@...emloft.net, saeedm@...dia.com, hch@....de,
        sagi@...mberg.me, axboe@...com, kbusch@...nel.org,
        viro@...iv.linux.org.uk, edumazet@...gle.com
Cc:     boris.pismenny@...il.com, linux-nvme@...ts.infradead.org,
        netdev@...r.kernel.org, benishay@...dia.com, ogerlitz@...dia.com,
        yorayz@...dia.com, Ben Ben-Ishay <benishay@...lanox.com>,
        Or Gerlitz <ogerlitz@...lanox.com>,
        Yoray Zack <yorayz@...lanox.com>
Subject: Re: [PATCH v1 net-next 01/15] iov_iter: Skip copy in memcpy_to_page
 if src==dst

On 12/7/20 2:06 PM, Boris Pismenny wrote:
> When using direct data placement the NIC writes some of the payload
> directly to the destination buffer, and constructs the SKB such that it
> points to this data. As a result, the skb_copy datagram_iter call will
> attempt to copy data when it is not necessary.
> 
> This patch adds a check to avoid this copy, and a static_key to enabled
> it when TCP direct data placement is possible.
> 

Why not mark the skb as ZEROCOPY -- an Rx version of the existing
SKBTX_DEV_ZEROCOPY and skb_shared_info->tx_flags? Use that as a generic
way of indicating to the stack what is happening.