linux-kernel - Re: [patch -mm 2/2] mempolicy: use default_policy mode instead of MPOL

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Date:	Mon, 10 Mar 2008 09:48:01 -0400
From:	Lee Schermerhorn <Lee.Schermerhorn@...com>
To:	Andi Kleen <ak@...e.de>
Cc:	David Rientjes <rientjes@...gle.com>, Paul Jackson <pj@....com>,
	akpm@...ux-foundation.org, clameter@....com,
	linux-kernel@...r.kernel.org
Subject: Re: [patch -mm 2/2] mempolicy: use default_policy mode instead of
	MPOL_DEFAULT

On Sun, 2008-03-09 at 00:19 +0100, Andi Kleen wrote:
> > Using MPOL_DEFAULT purely for falling back to the task or system-wide 
> > policy, however, seems confusing.  The semantics seem to indicate that 
> > MPOL_DEFAULT represents the system-wide default policy without any 
> > preferred node or set of nodes to bind or interleave.  So if a VMA has a 
> > policy of MPOL_DEFAULT then, to me, it seems like that indicates the 
> > absence of a specific policy, not a mandate to fallback to the task 
> > policy.
> 
> I designed MPOL_DEFAULT on vma originally to be a fallback to the task policy.
> 
> Absence of specific policy would be MPOL_PREFERRED with -1 node.

Not sure what you mean here, Andi.   

"MPOL_PREFERRED with -1 node" == "local allocation", right?  

Whereas, in the task mempolicy or vma policy or shared policy, the lack
of a specific policy--i.e., a null mempolicy pointer, or no policy at
that offset in a shared policy rbtree--means fall back to surrounding
context, right?  As far back as I've looked, mempolicy.c implemented
MPOL_DEFAULT, passed to set_mempolicy() or mbind(), by deleting the
target policy, resulting in "fall back".  

The only place that MPOL_DEFAULT actually occurs in a struct mempolicy
is in the system default policy.  I think we can simplify the code and
documentation--not have to explain the context dependent meaning of
MPOL_DEFAULT--by making it simply an API request to remove the target
policy and establish "default behavior" for that context--i.e.,
fallback.  

Lee
> 
> -Andi
> 
> 
> 

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