linux-kernel - Re: [PATCH -v2] rmap: make anon_vma_prepare link in all the anon

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Message-ID: <alpine.LFD.2.00.1004070951330.3487@i5.linux-foundation.org>
Date:	Wed, 7 Apr 2010 09:56:24 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Rik van Riel <riel@...hat.com>
cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Borislav Petkov <bp@...en8.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Minchan Kim <minchan.kim@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Nick Piggin <npiggin@...e.de>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	sgunderson@...foot.com, hannes@...xchg.org
Subject: Re: [PATCH -v2] rmap: make anon_vma_prepare link in all the anon_vmas
 of a mergeable VMA

On Wed, 7 Apr 2010, Rik van Riel wrote:
> 
> You are right, the idea was to continue use the locking that
> the anon_vma code was already using, without introducing any
> new locking with the anon_vma patches.
> 
> However, it has become clear that this is no longer possible,
> due to the need to hold a secondary lock across anon_vma_clone,
> when we come from a code path that holds the mmap_sem for read.

I do wonder if we could possibly simplify this a _lot_ by just requiring 
that the anon_vma gets allocated at vma creation time (ie mmap), rather 
than doing it on-demand when we actually do the page fault.

That would make all of this crap happen under mmap_sem held for writing, 
and it would simplify the faulting code (which is the much more critical 
code) a lot.

And it would make all your locking problems go away. Now all anon_vma code 
really _would_ run with mmap_sem held exclusively, without any races.

When I tried to do a "fill in multiple page table entries in one go" 
patch, that annoying anon_vma issue was a problem as well. Allocating the 
anon_vma up-front would have simplified that code too.

I can't imagine that we ever really have mappings without an anon_vma in 
practice _anyway_, so why delay the allocation until page fault time?

Maybe I'm missing something subtle. 

			Linus
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