linux-kernel - Re: [PATCH 1/3] mm,migration: During fork(), wait for migration to end if migration PTE is encountered

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Date:	Wed, 28 Apr 2010 00:22:45 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	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>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 1/3] mm,migration: During fork(), wait for migration to
 end if migration PTE is encountered

Ok I had a first look:

On Tue, Apr 27, 2010 at 10:30:50PM +0100, Mel Gorman wrote:
> 	CPUA			CPU B
> 				do_fork()
> 				copy_mm() (from process 1 to process2)
> 				insert new vma to mmap_list (if inode/anon_vma)

Insert to the tail of the anon_vma list...

> 	pte_lock(process1)
> 	unmap a page
> 	insert migration_entry
> 	pte_unlock(process1)
> 
> 	migrate page copy
> 				copy_page_range
> 	remap new page by rmap_walk()

rmap_walk will walk process1 first! It's at the head, the vmas with
unmapped ptes are at the tail so process1 is walked before process2.

> 	pte_lock(process2)
> 	found no pte.
> 	pte_unlock(process2)
> 				pte lock(process2)
> 				pte lock(process1)
> 				copy migration entry to process2
> 				pte unlock(process1)
> 				pte unlokc(process2)
> 	pte_lock(process1)
> 	replace migration entry
> 	to new page's pte.
> 	pte_unlock(process1)

rmap_walk has to lock down process1 before process2, this is the
ordering issue I already mentioned in earlier email. So it cannot
happen and this patch is unnecessary.

The ordering is fundamental and as said anon_vma_link already adds new
vmas to the _tail_ of the anon-vma. And this is why it has to add to
the tail. If anon_vma_link would add new vmas to the head of the list,
the above bug could materialize, but it doesn't so it cannot happen.

In mainline anon_vma_link is called anon_vma_chain_link, see the
list_add_tail there to provide this guarantee.

Because process1 is walked first by CPU A, the migration entry is
replaced by the final pte before copy-migration-entry
runs. Alternatively if copy-migration-entry runs before before
process1 is walked, the migration entry will be copied and found in
process 2.

Comments welcome.
Andrea
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