linux-kernel - Re: [RFC][PATCH 6/8] mm: handle_speculative

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Date:	Tue, 05 Jan 2010 09:57:14 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"minchan.kim@...il.com" <minchan.kim@...il.com>,
	cl@...ux-foundation.org,
	"hugh.dickins" <hugh.dickins@...cali.co.uk>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [RFC][PATCH 6/8] mm: handle_speculative_fault()

On Mon, 2010-01-04 at 19:13 -0800, Linus Torvalds wrote:
> Or put another way: if the vma was a writable mapping, a user may do
> 
>         munmap(mapping, size);
> 
> and the backing file is still active and writable AFTER THE MUNMAP! This 
> can be a huge problem for something that wants to unmount the volume, for 
> example, or depends on the whole writability-vs-executability thing. The 
> user may have unmapped it, and expects the file to be immediately 
> non-busy, but with the delayed free that isn't the case any more. 

If it were only unmount it would be rather easy to fix by putting that
RCU synchronization in unmount, unmount does a lot of sync things
anyway. But I suspect there's more cases where that non-busy matters
(but I'd need to educate myself on filesystems/vfs to come up with any).

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