Message-ID: <fe8252bd-17bd-850d-dcd0-d799443681e9@kernel.dk>
Date:   Fri, 10 Feb 2023 13:39:43 -0700
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Subject: Re: copy on write for splice() from file to pipe?
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To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Dave Chinner <david@fromorbit.com>,
        Matthew Wilcox <willy@infradead.org>,
        Stefan Metzmacher <metze@samba.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux API Mailing List <linux-api@vger.kernel.org>,
        io-uring <io-uring@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Samba Technical <samba-technical@lists.samba.org>
References: <0cfd9f02-dea7-90e2-e932-c8129b6013c7@samba.org>
 <CAHk-=wj8rthcQ9gQbvkMzeFt0iymq+CuOzmidx3Pm29Lg+W0gg@mail.gmail.com>
 <20230210021603.GA2825702@dread.disaster.area>
 <20230210040626.GB2825702@dread.disaster.area>
 <Y+XLuYh+kC+4wTRi@casper.infradead.org>
 <20230210065747.GD2825702@dread.disaster.area>
 <CALCETrWjJisipSJA7tPu+h6B2gs3m+g0yPhZ4z+Atod+WOMkZg@mail.gmail.com>
 <CAHk-=wj66F6CdJUAAjqigXMBy7gHquFMzPNAwKCgkrb2mF6U7w@mail.gmail.com>
 <CALCETrU-9Wcb_zCsVWr24V=uCA0+c6x359UkJBOBgkbq+UHAMA@mail.gmail.com>
 <CAHk-=wjQZWMeQ9OgXDNepf+TLijqj0Lm0dXWwWzDcbz6o7yy_g@mail.gmail.com>
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 <CAHk-=wiszt6btMPeT5UFcS=0=EVr=0injTR75KsvN8WetwQwkA@mail.gmail.com>
From:   Jens Axboe <axboe@kernel.dk>
In-Reply-To: <CAHk-=wiszt6btMPeT5UFcS=0=EVr=0injTR75KsvN8WetwQwkA@mail.gmail.com>
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On 2/10/23 1:36 PM, Linus Torvalds wrote:
> On Fri, Feb 10, 2023 at 12:32 PM Jens Axboe <axboe@kernel.dk> wrote:
>>
>> No, we very much do have that for io_uring zerocopy sends, which was in
>> the bit below you snipped from the reply. It'll tell you when data has
>> been sent out, and when the data has been acked.
> 
> Hmm. splice() itself definitely doesn't have that data - there's no
> "io context" for it.
> 
> There is only the pipe buffers, and they are released when the data
> has been accepted - which is not the same as used (eg the networking
> layer just takes another ref to the page and says "I'm done").
> 
> Maybe adding some io context to the pipe buffer would be possible, but
> it's certainly not obvious how, without changing splice() semantics
> completely.

Right, I'm referencing doing zerocopy data sends with io_uring, using
IORING_OP_SEND_ZC. This isn't from a file, it's from a memory location,
but the important bit here is the split notifications and how you
could wire up a OP_SENDFILE similarly to what Andy described.

-- 
Jens Axboe