linux-kernel - Re: [PATCH 1/2] mm,migration: Prevent rmap_walk_[anon|ksm] seeing the wrong VMA information

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Date:	Thu, 6 May 2010 07:06:08 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Minchan Kim <minchan.kim@...il.com>
cc:	Mel Gorman <mel@....ul.ie>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Christoph Lameter <cl@...ux.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 1/2] mm,migration: Prevent rmap_walk_[anon|ksm] seeing
 the  wrong VMA information

On Thu, 6 May 2010, Minchan Kim wrote:
> > +        */
> > +       avc = list_first_entry(&anon_vma->head, struct anon_vma_chain, same_anon_vma);
> 
> Dumb question.
> 
> I can't understand why we should use list_first_entry.

It's not that we "should" use list_entry_first. It's that we want to find 
_any_ entry on the list, and the most natural one is the first one.

So we could take absolutely any 'avc' entry that is reachable from the 
anon_vma, and use that to look up _any_ 'vma' that is associated with that 
anon_vma. And then, from _any_ of those vma's, we know how to get to the 
"root anon_vma" - the one that they are all associated with.

So no, there's absolutely nothing special about the first entry. It's 
just a random easily found one.

		Linus
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