linux-kernel - Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped anonymous pages

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Date:	Wed, 17 Mar 2010 13:19:12 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Mel Gorman <mel@....ul.ie>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	David Rientjes <rientjes@...gle.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 02/11] mm,migration: Do not try to migrate unmapped 
 anonymous pages

On Wed, 17 Mar 2010 13:15:14 +0900
Minchan Kim <minchan.kim@...il.com> wrote:

> On Wed, Mar 17, 2010 at 12:15 PM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@...fujitsu.com> wrote:
> > On Wed, 17 Mar 2010 12:00:15 +0900
> > Minchan Kim <minchan.kim@...il.com> wrote:
> >
> >> On Wed, Mar 17, 2010 at 11:12 AM, KAMEZAWA Hiroyuki
> >> > BTW, I doubt freeing anon_vma can happen even when we check mapcount.
> >> >
> >> > "unmap" is 2-stage operation.
> >> >        1. unmap_vmas() => modify ptes, free pages, etc.
> >> >        2. free_pgtables() => free pgtables, unlink vma and free it.
> >> >
> >> > Then, if migration is enough slow.
> >> >
> >> >        Migration():                            Exit():
> >> >        check mapcount
> >> >        rcu_read_lock
> >> >        pte_lock
> >> >        replace pte with migration pte
> >> >        pte_unlock
> >> >                                                pte_lock
> >> >        copy page etc...                        zap pte (clear pte)
> >> >                                                pte_unlock
> >> >                                                free_pgtables
> >> >                                                ->free vma
> >> >                                                ->free anon_vma
> >> >        pte_lock
> >> >        remap pte with new pfn(fail)
> >> >        pte_unlock
> >> >
> >> >        lock anon_vma->lock             # modification after free.
> >> >        check list is empty
> >>
> >> check list is empty?
> >> Do you mean anon_vma->head?
> >>
> > yes.
> >
> >> If it is, is it possible that that list isn't empty since anon_vma is
> >> used by others due to
> >> SLAB_DESTROY_BY_RCU?
> >>
> > There are 4 cases.
> >        A) anon_vma->list is not empty because anon_vma is not freed.
> >        B) anon_vma->list is empty because it's freed.
> >        C) anon_vma->list is empty but it's reused.
> >        D) anon_vma->list is not empty but it's reused.
> 
> E) anon_vma is used for other object.
> 
> That's because we don't hold rcu_read_lock.
> I think Mel met this E) situation.
> 
Hmm. 

> AFAIU, even slab page of SLAB_BY_RCU can be freed after grace period.
> Do I miss something?
> 
I miss something. Sorry for noise.

Maybe we need check page_mapped() before calling try_to_unmap() as
vmscan does. Thank you for your help.

Thanks,
-Kame

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