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

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Date:	Wed, 14 Apr 2010 16:27:08 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Johannes Weiner <hannes@...xchg.org>,
	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
Subject: Re: [PATCH -v2] rmap: make anon_vma_prepare link in all the
 anon_vmas of a mergeable VMA

On Tue, 2010-04-13 at 21:00 +0900, KOSAKI Motohiro wrote:
> > [ with the preemptible mmu_gather patches I introduce a refcount to
> >   the anon_vma, and then with atomic_inc_not_zero() we can add a
> >   guarantee that the returned anon_vma is alive ]
> 
> Indeed. refcount is best way. anon_vma DESTROY_BY_RCU stuff seems
> overengineering, I think. this is fastest, but anon_vma allocation is not
> (and was not) fork/exit bottleneck point. So, I guess most simply way is
> best. 

Well, that refcount stuff still relies on DESTROY_BY_RCU :-)

Anyway, it also looks like a lot of races are avoided by ordering the
rmap_add/remove calls wrt to adding/removing the page to/from the LRU.

Rmap calls come from LRU pages, and it looks like rmap state is only
changed for pages that are not on the LRU.

I still have to go through all that code again to make sure, but I
couldn't find a race between page_add_anon_rmap() and
page_lock_anon_vma() due to that.

If there is, we need to look at page_mapped() before page->mapping
because page_add_anon_rmap() first increments the mapcount and only then
adjusts the mapping, so the existing order in page_anon_lock_vma() can
end up dereferencing a long dead anon_vma.

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