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Date: Fri, 8 Dec 2006 15:45:22 -0800
From: Andrew Morton <akpm@osdl.org>
Subject: Re: [PATCH RFC] use of activate_mm in fs/aio.c:use_mm()?
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To: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Zach Brown <zach.brown@oracle.com>, Badari Pulavarty <pbadari@us.ibm.com>, Nick Piggin <nickpiggin@yahoo.com.au>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Zachary Amsden <zach@vmware.com>, Chris Wright <chrisw@sous-sol.org>, Rusty Russell <rusty@rustcorp.com.au>, Jeff Dike <jdike@addtoit.com>, Linus Torvalds <torvalds@osdl.org>, linux-arch@vger.kernel.org, linux-aio@kvack.org
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On Wed, 06 Dec 2006 17:36:02 -0800
Jeremy Fitzhardinge <jeremy@goop.org> wrote:

> Jeremy Fitzhardinge wrote:
> > I'm wondering if activate_mm() is the right thing to be using in
> > use_mm(); shouldn't this be switch_mm()?
> >
> > On normal x86, they're synonymous, but for the Xen patches I'm adding a
> > hook which assumes that activate_mm is only used the first time a new mm
> > is used after creation (I have another hook for dealing with dup_mm).  I
> > think this use of activate_mm() is the only place where it could be used
> > a second time on an mm.
> >
> > From a quick look at the other architectures I think this is OK (most
> > simply implement one in terms of the other), but some are doing some
> > subtly different stuff between the two.
> >
> > Thanks,
> >     J
> >
> >
> >   
> Er, lets try that again:
> 
> diff -r 455b71ed4525 fs/aio.c
> --- a/fs/aio.c	Wed Dec 06 13:16:42 2006 -0800
> +++ b/fs/aio.c	Wed Dec 06 17:17:43 2006 -0800
> @@ -588,7 +588,7 @@ static void use_mm(struct mm_struct *mm)
>  	 * Note that on UML this *requires* PF_BORROWED_MM to be set, otherwise
>  	 * it won't work. Update it accordingly if you change it here
>  	 */
> -	activate_mm(active_mm, mm);
> +	switch_mm(active_mm, mm, tsk);
>  	task_unlock(tsk);
>  
>  	mmdrop(active_mm);

That to me sounds like a reasonable description of the difference between
activate_mm() and switch_mm().  And the change appears reasonable as well.

But it is a change which the architecture maintainers would need to have a
think about, please.