netdev - Re: [bpf-next PATCH v2 06/18] bpf: sockmap, add bpf_msg_apply

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Message-ID: <20180315214554.2gspfp255qqxbjcb@ast-mbp>
Date:   Thu, 15 Mar 2018 14:45:56 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     davem@...emloft.net, ast@...nel.org, daniel@...earbox.net,
        davejwatson@...com, netdev@...r.kernel.org
Subject: Re: [bpf-next PATCH v2 06/18] bpf: sockmap, add
 bpf_msg_apply_bytes() helper

On Mon, Mar 12, 2018 at 12:23:34PM -0700, John Fastabend wrote:
> A single sendmsg or sendfile system call can contain multiple logical
> messages that a BPF program may want to read and apply a verdict. But,
> without an apply_bytes helper any verdict on the data applies to all
> bytes in the sendmsg/sendfile. Alternatively, a BPF program may only
> care to read the first N bytes of a msg. If the payload is large say
> MB or even GB setting up and calling the BPF program repeatedly for
> all bytes, even though the verdict is already known, creates
> unnecessary overhead.
> 
> To allow BPF programs to control how many bytes a given verdict
> applies to we implement a bpf_msg_apply_bytes() helper. When called
> from within a BPF program this sets a counter, internal to the
> BPF infrastructure, that applies the last verdict to the next N
> bytes. If the N is smaller than the current data being processed
> from a sendmsg/sendfile call, the first N bytes will be sent and
> the BPF program will be re-run with start_data pointing to the N+1
> byte. If N is larger than the current data being processed the
> BPF verdict will be applied to multiple sendmsg/sendfile calls
> until N bytes are consumed.
> 
> Note1 if a socket closes with apply_bytes counter non-zero this
> is not a problem because data is not being buffered for N bytes
> and is sent as its received.
> 
> Note2 if this is operating in the sendpage context the data
> pointers may be zeroed after this call if the apply walks beyond
> a msg_pull_data() call specified data range. (helper implemented
> shortly in this series).

instead of 'shortly in this seris' you meant 'implemented earlier'?
patch 5 handles it, but it's set here, right?

The semantics of the helper looks great.