linux-kernel - Re: [patch v3] splice: fix race with page invalidation

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Message-ID: <alpine.LFD.1.10.0807301751320.3277@nehalem.linux-foundation.org>
Date:	Wed, 30 Jul 2008 17:54:20 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jamie Lokier <jamie@...reable.org>
cc:	Miklos Szeredi <miklos@...redi.hu>, jens.axboe@...cle.com,
	akpm@...ux-foundation.org, nickpiggin@...oo.com.au,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [patch v3] splice: fix race with page invalidation

On Wed, 30 Jul 2008, Linus Torvalds wrote:
> 
> Personally, I think the right approach is to just realize that splice() is 
> _not_ a write() system call, and never will be. If you need synchronous 
> writing, you simply shouldn't use splice().

Side note: in-kernel users could probably do somethign about this. IOW, if 
there's some in-kernel usage (and yes, knfsd would be a prime example), 
that one may actually be able to do things that a _user_level user of 
splice() could never do.

That includes things like getting the inode semaphore over a write (so 
that you can guarantee that pages that are in flight are not modified, 
except again possibly by other mmap users), and/or a per-page callback for 
when splice() is done with a page (so that you could keep the page locked 
while it's getting spliced, for example).

And no, we don't actually have that either, of course.

			Linus
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