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From: Andy Lutomirski <luto@kernel.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
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, USB list <linux-usb@vger.kernel.org>,
	linux-block <linux-block@vger.kernel.org>,
	LSM List <linux-security-module@vger.kernel.org>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH 11/10] pipe: Add fsync() support [ver #2]
Date: Sat, 2 Nov 2019 16:14:45 -0700	[thread overview]
Message-ID: <CALCETrWZjW88OY2mh7v8cUU_6XTSJTkQhAfNbSC17AdhEWwVAA@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=wgPQutQ8d8kUCvAFi+hfNWgaNLiZPkbg-GXY2DCtD-Z5Q@mail.gmail.com>

On Sat, Nov 2, 2019 at 4:10 PM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> 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.

I suppose a nicer interface might be:


madvise(buf, len, MADV_STABILIZE);

(MADV_STABILIZE is an imaginary operation that write protects the
memory a la fork() but without the copying part.)

vmsplice_safer(fd, ...);

Where vmsplice_safer() is like vmsplice, except that it only works on
write-protected pages.  If you vmsplice_safer() some memory and then
write to the memory, the pipe keeps the old copy.

But this can all be done with memfd and splice, too, I think.


>
> 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.

I think this is the right choice.

FWIW, the openssl vmsplice() call looks dubious, but I suspect it's
okay because it's vmsplicing to a netlink socket, and the kernel code
on the other end won't read the data after it returns a response.

--Andy

  reply	other threads:[~2019-11-02 23:15 UTC|newest]

Thread overview: 52+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-23 20:17 [RFC PATCH 00/10] pipe: Notification queue preparation [ver #2] David Howells
2019-10-23 20:17 ` [RFC PATCH 01/10] pipe: Reduce #inclusion of pipe_fs_i.h " David Howells
2019-10-23 20:17 ` [RFC PATCH 02/10] Remove the nr_exclusive argument from __wake_up_sync_key() " David Howells
2019-10-23 20:17 ` [RFC PATCH 03/10] Add wake_up_interruptible_sync_poll_locked() " David Howells
2019-10-23 20:17 ` [RFC PATCH 04/10] pipe: Use head and tail pointers for the ring, not cursor and length " David Howells
2019-10-27 14:03   ` Linus Torvalds
2019-10-30 16:19   ` Ilya Dryomov
2019-10-30 20:35     ` Rasmus Villemoes
2019-10-30 22:16       ` Ilya Dryomov
2019-10-30 22:38         ` Rasmus Villemoes
2019-10-31 15:11     ` David Howells
2019-10-31 15:57       ` Ilya Dryomov
2019-11-01 14:53       ` David Howells
2019-10-31 14:57   ` David Howells
2019-11-03 11:17     ` Matthew Wilcox
2019-12-06 21:47   ` Johannes Hirte
2019-12-06 22:14     ` Linus Torvalds
2019-12-07  0:00       ` Johannes Hirte
2019-12-07  1:03         ` Linus Torvalds
2019-12-08 17:56           ` Johannes Hirte
2019-12-08 18:10             ` Linus Torvalds
2019-12-07  6:47         ` Linus Torvalds
2019-12-06 22:15   ` David Howells
2019-10-23 20:17 ` [RFC PATCH 05/10] pipe: Allow pipes to have kernel-reserved slots " David Howells
2019-10-23 20:18 ` [RFC PATCH 06/10] pipe: Advance tail pointer inside of wait spinlock in pipe_read() " David Howells
2019-10-23 20:18 ` [RFC PATCH 07/10] pipe: Conditionalise wakeup " David Howells
2019-10-27 15:57   ` Konstantin Khlebnikov
2019-10-31 15:21   ` David Howells
2019-10-31 16:38   ` David Howells
2019-11-03 11:04     ` Konstantin Khlebnikov
2019-10-23 20:18 ` [RFC PATCH 08/10] pipe: Rearrange sequence in pipe_write() to preallocate slot " David Howells
2019-10-23 20:18 ` [RFC PATCH 09/10] pipe: Remove redundant wakeup from pipe_write() " David Howells
2019-10-23 20:18 ` [RFC PATCH 10/10] pipe: Check for ring full inside of the spinlock in " David Howells
2019-10-24 10:32 ` [RFC PATCH 04/10] pipe: Use head and tail pointers for the ring, not cursor and length " David Howells
2019-10-24 13:14 ` [RFC PATCH 00/10] pipe: Notification queue preparation " Peter Zijlstra
2019-10-24 16:57 ` [RFC PATCH 11/10] pipe: Add fsync() support " David Howells
2019-10-24 21:29   ` Linus Torvalds
2019-10-25  8:34   ` David Howells
2019-10-27 15:22   ` Christoph Hellwig
2019-10-27 16:04   ` Konstantin Khlebnikov
2019-10-31 15:13   ` David Howells
2019-10-31 15:15   ` David Howells
2019-11-02 18:53     ` Linus Torvalds
2019-11-02 19:34     ` David Howells
2019-11-02 20:31       ` Andy Lutomirski
2019-11-02 22:03         ` Linus Torvalds
2019-11-02 22:09           ` Linus Torvalds
2019-11-02 22:30           ` Andy Lutomirski
2019-11-02 23:02             ` Linus Torvalds
2019-11-02 23:09               ` Linus Torvalds
2019-11-02 23:14                 ` Andy Lutomirski [this message]
2019-11-03 12:02                   ` Konstantin Khlebnikov

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