linux-kernel - Re: [PATCH 1/3] revoke: misc fixes

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Message-ID: <45FA4984.9000506@yahoo.com.au>
Date:	Fri, 16 Mar 2007 18:38:44 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Pekka J Enberg <penberg@...helsinki.fi>
CC:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] revoke: misc fixes

Pekka J Enberg wrote:
> Pekka J Enberg wrote:
> 
>>>	/*
>>>- 	 * Not holding ->mmap_sem here.
>>>+ 	 * Not holding ->mmap_sem here but we must watch out for page
>>>+ 	 * faults and after the shared mappings have been taken down
>>>+ 	 * and sys_mmap() trying to remap the revoked range.
>>>  */
>>>  vma->vm_flags |= VM_REVOKED;
>>>  smp_mb();
>>>@@ -455,7 +457,7 @@ 	int err = 0;
> 
> 
> On Fri, 16 Mar 2007, Nick Piggin wrote:
> 
>>You're still modifying vm_flags without down_write mmap_sem, so this will
>>corrupt vm_flags.
> 
> 
> Uhm, you're right, two concurrent writes and we can lose some bits so a 
> barrier doesn't work. Too bad as we're under mapping->i_mmap_lock here and 
> thus cannot take ->mmap_sep...
> 

Could you try something like walk the i_mmap lists to find mms with vmas that
haven't need revoking, then each time you find one, take a ref on the mm, drop
i_mmap_lock, take mmap_sem, and walk all its vmas looking for any that reference
the inode?

Bit of a roundabout way to go, but it might work.

-- 
SUSE Labs, Novell Inc.
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