linux-kernel - Re: [RFC PATCH 11/10] pipe: Add fsync() support [ver #2]

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Message-Id: <CC3328CC-2F05-461E-AAC3-8DEBAB1BA162@amacapital.net>
Date:   Sat, 2 Nov 2019 13:31:10 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     David Howells <dhowells@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>, raven@...maw.net,
        Christian Brauner <christian@...uner.io>,
        keyrings@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-block <linux-block@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 11/10] pipe: Add fsync() support [ver #2]

> On Nov 2, 2019, at 12:34 PM, David Howells <dhowells@...hat.com> wrote:
> 
> Linus Torvalds <torvalds@...ux-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”.