linux-kernel - Re: [PATCH] Define a UNIQUE value for AS

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Message-ID: <49D50E4E.20103@redhat.com>
Date:	Thu, 02 Apr 2009 22:13:18 +0300
From:	Avi Kivity <avi@...hat.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	stable <stable@...nel.org>, Rik van Riel <riel@...hat.com>,
	David Howells <dhowells@...hat.com>
Subject: Re: [PATCH] Define a UNIQUE value for AS_UNEVICTABLE flag

KOSAKI Motohiro wrote:
> (cc to Avi)
>
>   
>>> [PATCH] Define UNIQUE value of AS_UNEVICTABLE
>>>
>>> Needed in 2.6.28, 2.6.29, ...
>>>
>>> A new "address_space flag"--AS_MM_ALL_LOCKS--was defined to use the next
>>> available AS flag while the Unevictable LRU was under development.  The
>>> Unevictable LRU was using the same flag and "no one" noticed.  Current
>>> mainline, since 2.6.28, has same value for two symbolic flag names.
>>>       
>> argh.
>>
>> What are the user-observable effects of the bug, and why didn't anyone
>> notice it until now?
>>     
>
> AS_MM_ALL_LOCKS is used by mmu_notifier. it mean it is used by only kvm.
> In the other hand,  AS_UNEVICTABLE mean unevictable shmem or ramfs.
>
> Then, if shmem opend process use ioctl(KVM_CREATE_VM), unevictable
> flag on the shmem accidentally turn off.
> but, fortunatelly, In modern desktop environment, only KVM control
> program use above ioctl. then, we can assume this doesn't use shmem
> and ramfs.
>
> Am I missing anything?
>   

Qemu, the main kvm user, indeed doesn't use shm or ramfs.  However it is 
not inconceivable that one day it will, as a way to share memory among 
guests.

It isn't surprising that no bug was observed, but let's fix it.

(If hugetlbfs is considered unevictable, then qemu can trigger this bug 
today)

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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