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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sat, 2 Nov 2019 16:09:55 -0700
Message-ID: <CAHk-=wgPQutQ8d8kUCvAFi+hfNWgaNLiZPkbg-GXY2DCtD-Z5Q@mail.gmail.com>
Subject: Re: [RFC PATCH 11/10] pipe: Add fsync() support [ver #2]
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     David Howells <dhowells@redhat.com>,
        Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Nicolas Dichtel <nicolas.dichtel@6wind.com>, raven@themaw.net,
        Christian Brauner <christian@brauner.io>,
        keyrings@vger.kernel.org, linux-usb@vger.kernel.org,
        linux-block <linux-block@vger.kernel.org>,
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On Sat, Nov 2, 2019 at 4:02 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> But I don't think anybody actually _did_ any of that. But that's
> basically the argument for the three splice operations:
> write/vmsplice/splice(). Which one you use depends on the lifetime and
> the source of your data. write() is obviously for the copy case (the
> source data might not be stable), while splice() is for the "data from
> another source", and vmsplace() is "data is from stable data in my
> vm".

Btw, it's really worth noting that "splice()" and friends are from a
more happy-go-lucky time when we were experimenting with new
interfaces, and in a day and age when people thought that interfaces
like "sendpage()" and zero-copy and playing games with the VM was a
great thing to do.

It turns out that VM games are almost always more expensive than just
copying the data in the first place, but hey, people didn't know that,
and zero-copy was seen a big deal.

The reality is that almost nobody uses splice and vmsplice at all, and
they have been a much bigger headache than they are worth. If I could
go back in time and not do them, I would. But there have been a few
very special uses that seem to actually like the interfaces.

But it's entirely possible that we should kill vmsplice() (likely by
just implementing the semantics as "write()") because it's not common
enough to have the complexity.

             Linus