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From: Andy Lutomirski <luto@amacapital.net>
To: David Howells <dhowells@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	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>,
	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 13:31:10 -0700	[thread overview]
Message-ID: <CC3328CC-2F05-461E-AAC3-8DEBAB1BA162@amacapital.net> (raw)
In-Reply-To: <25886.1572723272@warthog.procyon.org.uk>



> On Nov 2, 2019, at 12:34 PM, David Howells <dhowells@redhat.com> wrote:
> 
> Linus Torvalds <torvalds@linux-foundation.org> wrote:
> 
>>>> Similar synchronization is required for reusing memory after vmsplice()?
>>>> I don't see other way how sender could safely change these pages.
> 
> Actually, it's probably worse than that.  If the output of the pipe gets teed
> or spliced somewhere else, you still don't know when the vmspliced pages are
> finished with.
> 
> 

I sometimes wonder whether vmsplice should be disallowed or severely restricted. Even ignoring these usability issues, it makes me very uncomfortable that you can have some data queue up on a pipe, tee() it, and get *different* data in the original pipe and the teed copy because the sender used vmsplice and is messing with you.

Add in the fact that it’s not obvious that vmsplice *can* be used correctly, and I’m wondering if we should just remove it or make it just do write() under the hood.

I suppose the kernel could guarantee that it stops referring to the vmsplice source pages as soon as anything sees *or* tees the data. This way it would be, at least in principle, possible to say “hey, the pipe has consumed the first n vmspliced bytes, so I can reuse that memory”.

  reply	other threads:[~2019-11-02 20:31 UTC|newest]

Thread overview: 53+ 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
2019-10-26 14:58   ` [pipe] 6567a02d20: BUG:kernel_NULL_pointer_dereference,address kernel test robot
2019-10-27 14:03   ` [RFC PATCH 04/10] pipe: Use head and tail pointers for the ring, not cursor and length [ver #2] 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 [this message]
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
2019-11-03 12:02                   ` Konstantin Khlebnikov

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