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

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Message-ID: <28c262361003162000w34cc13ecnbd32840a0df80f95@mail.gmail.com>
Date:	Wed, 17 Mar 2010 12:00:15 +0900
From:	Minchan Kim <minchan.kim@...il.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.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, 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?

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?

but such case is handled by page_check_address, vma_address, I think.

>        unlock anon_vma->lock
>        free anon_vma
>        rcu_read_unlock
>
>
> Hmm. IIUC, anon_vma is allocated as SLAB_DESTROY_BY_RCU. Then, while
> rcu_read_lock() is taken, anon_vma is anon_vma even if freed. But it
> may reused as anon_vma for someone else.
> (IOW, it may be reused but never pushed back to general purpose memory
>  until RCU grace period.)
> Then, touching anon_vma->lock never cause any corruption.
>
> Does use-after-free check for SLAB_DESTROY_BY_RCU correct behavior ?

Could you elaborate your point?

> Above case is not use-after-free. It's safe and expected sequence.
>
> Thanks,
> -Kame
>
>
>
>> > ---
>> >  mm/migrate.c |   13 +++++++++++++
>> >  1 files changed, 13 insertions(+), 0 deletions(-)
>> >
>> > diff --git a/mm/migrate.c b/mm/migrate.c
>> > index 98eaaf2..6eb1efe 100644
>> > --- a/mm/migrate.c
>> > +++ b/mm/migrate.c
>> > @@ -603,6 +603,19 @@ static int unmap_and_move(new_page_t get_new_page, unsigned long private,
>> >      */
>> >     if (PageAnon(page)) {
>> >             rcu_read_lock();
>> > +
>> > +           /*
>> > +            * If the page has no mappings any more, just bail. An
>> > +            * unmapped anon page is likely to be freed soon but worse,
>> > +            * it's possible its anon_vma disappeared between when
>> > +            * the page was isolated and when we reached here while
>> > +            * the RCU lock was not held
>> > +            */
>> > +           if (!page_mapcount(page)) {
>> > +                   rcu_read_unlock();
>> > +                   goto uncharge;
>> > +           }
>> > +
>> >             rcu_locked = 1;
>> >             anon_vma = page_anon_vma(page);
>> >             atomic_inc(&anon_vma->migrate_refcount);
>> >
>>
>> --
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>>
>
>



-- 
Kind regards,
Minchan Kim
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