linux-kernel - Re: [PATCH] Fix OOPS in mmap_region() when merging adjacent VM

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Date:	Thu, 29 Jan 2009 14:47:05 -0800
From:	Maksim Yevmenkin <maksim.yevmenkin@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...e.de>, will@...wder-design.com,
	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH] Fix OOPS in mmap_region() when merging adjacent VM_LOCKED 
	file segments

On Thu, Jan 29, 2009 at 12:48 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Thu, 29 Jan 2009, Linus Torvalds wrote:
>>
>> THIS PATCH IS TOTALLY UNTESTED!
>
> Well, it boots. FWIW. I've not really tested anything interesting with it,
> but any potential breakage is at least not catastrophic and immediate.
>
>> diff --git a/mm/mmap.c b/mm/mmap.c
>> index 8d95902..3f78ead 100644
>> --- a/mm/mmap.c
>> +++ b/mm/mmap.c
>> @@ -769,6 +769,10 @@ struct vm_area_struct *vma_merge(struct mm_struct *mm,
>>       if (vm_flags & VM_SPECIAL)
>>               return NULL;
>>
>> +     /* Anonymous shared mappings are unsharable */
>> +     if ((vm_flags & VM_SHARED) && !file)
>> +             return NULL;
>> +
>
> .. and I think this part of it is actually unnecessary, because what
> happens is that a shared anon mapping is turned into a shmem mapping when
> it is inserted, and that actually ends up allocating a file for it. So the
> vma->vm_file for anon mappings will not match a NULL file pointer
> _anyway_, so there's no way it would end up merging.
>
> So my patch can be further simplified, I think, to just the following.
> Even more total lines removed.
>
> I still want somebody else to look at and think about it, though.

Just to confirm. This patch also appear to fix the immediate issue for us.

thanks,
max
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