linux-kernel - Re: Ugly rmap NULL ptr deref oopsie on hibernate (was Linux 2.6.34-rc3)

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Date:	Tue, 06 Apr 2010 13:04:41 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Minchan Kim <minchan.kim@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Borislav Petkov <bp@...en8.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Nick Piggin <npiggin@...e.de>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>
Subject: Re: Ugly rmap NULL ptr deref oopsie on hibernate (was Linux 2.6.34-rc3)

On 04/06/2010 12:53 PM, Linus Torvalds wrote:
> On Wed, 7 Apr 2010, Minchan Kim wrote:
>>
>> unmap_and_move
>> 	remove_migration_ptes
>> 		rmap_walk
>> 			rmap_walk_anon
>>
>> We always has rcu_read_lock about anon page in unmap_and_move.
>> So I think it's not buggy. What am I missing?
>
> Ok, in that case it's fine.
>
> However, it does bring back my comment about all those anonvma changes:
> the locking is totally undocumented.
>
> Why isn't there a thing _saying_ that it's ok because of this?
>
> Why is there no comment about the locking of that 'same_vma' /
> 'vma->anon_vma_chain' except for the totally nonsensical one about
> page_table_lock (which doesn't protect _any_ of the other cases)?

Which other cases? When do we ever walk the "same_vma" list
not from the context of the process owning the vma?

This bug in page_referenced is walking the "same_anon_vma" list,
which is locked with the anon_vma->lock.
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