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

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Message-ID: <fab7b6a0-c84a-a715-3516-5efe474c2dea@gmail.com>
Date:   Thu, 15 Mar 2018 14:59:33 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...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 03/15/2018 02:45 PM, Alexei Starovoitov wrote:
> 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?
> 

Yep just a hold-over from an earlier patch description. I'll remove
that entire note2 and fixup a couple small things Daniel noticed
with a v3.

> The semantics of the helper looks great.
> 

Great!