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 11:37:05 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Minchan Kim <minchan.kim@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Christoph Lameter <cl@...ux.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 Wed, May 05, 2010 at 06:13:19PM +0200, Andrea Arcangeli wrote:
> On Wed, May 05, 2010 at 04:54:54PM +0100, Mel Gorman wrote:
> > I'm still thinking of the ordering but one possibility would be to use a mutex
> 
> I can't take mutex in split_huge_page... so I'd need to use an other solution.
> 
> > Not yet.
> 
> Rik's patch that takes the locks in the faster path is preferable to
> me, it's just simpler, you know the really "strong" long is the
> page->mapping/anon_vma->lock and nothing else.

The hatchet-job mutex is off the table so it's down to

start-with-root-anon_vma-and-lock-in-order-when-walking-list (what I last posted)
take-all-anon_vma-locks-when-changing-vmas (Rik's)
use-seq-counter-to-spot-changes-to-VMAs-when-walking-list (Kamezawa-san's approach)

Any strong preference?

I still haven't read the other comments Linus made so I don't have a strong
preference yet. Either Rik's or the patch I posted should be enough for
migration to not get tripped up as far as I can see.

> You've a page, you take
> that lock, you're done for that very page.
> 
> Sure that means updating vm_start/vm_pgoff then requires locking all
> anon_vmas that the vma registered into, but that's conceptually
> simpler and it doesn't alter the page_lock_anon_vma semantics. Now I
> wonder if you said the same_anon_vma is in order, but the same_vma is
> not, if it's safe to lock the same_vma in list order in anon_vma_lock,
> I didn't experience problems on the anon_vma_chain branch but
> anon_vma_lock disables all lockdep lock inversion checking.
> 

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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