Re: [PATCH/RFC 2/5] Mem Policy: Use MPOL_PREFERRED for system-wide default policy

[Lee Schermerhorn <Lee.Schermerhorn@hp.com>]

On Tue, 2007-09-11 at 19:54 +0100, Mel Gorman wrote:
> On Thu, 2007-08-30 at 14:51 -0400, Lee Schermerhorn wrote:
> > PATCH/RFC 2/5 Use MPOL_PREFERRED for system-wide default policy
> > 
> > Against:  2.6.23-rc3-mm1
> > 
> > V1 -> V2:
> > + restore BUG()s in switch(policy) default cases -- per
> >   Christoph
> > + eliminate unneeded re-init of struct mempolicy policy member
> >   before freeing
> > 
> > Currently, when one specifies MPOL_DEFAULT via a NUMA memory
> > policy API [set_mempolicy(), mbind() and internal versions],
> > the kernel simply installs a NULL struct mempolicy pointer in
> > the appropriate context:  task policy, vma policy, or shared
> > policy.  This causes any use of that policy to "fall back" to
> > the next most specific policy scope.  The only use of MPOL_DEFAULT
> > to mean "local allocation" is in the system default policy.
> > 
> 
> In general, this seems like a good idea. It's certainly simplier to
> always assume a policy exists because it discourages "bah, I don't care
> about policies" style of thinking.

More importantly, IMO, it eliminates 2 meanings for MPOL_DEFAULT in
different contexts and promotes the use 0f [MPOL_PREFERRED,
-1/null-nodemask] for local allocation.  I think this makes the
resulting documentation clearer.

<snip>
> > 
> > Index: Linux/mm/mempolicy.c
> > ===================================================================
> > --- Linux.orig/mm/mempolicy.c	2007-08-29 11:43:06.000000000 -0400
> > +++ Linux/mm/mempolicy.c	2007-08-29 11:44:03.000000000 -0400
> > @@ -105,9 +105,13 @@ static struct kmem_cache *sn_cache;
> >     policied. */
> >  enum zone_type policy_zone = 0;
> >  
> > +/*
> > + * run-time system-wide default policy => local allocation
> > + */
> >  struct mempolicy default_policy = {
> >  	.refcnt = ATOMIC_INIT(1), /* never free it */
> > -	.policy = MPOL_DEFAULT,
> > +	.policy = MPOL_PREFERRED,
> > +	.v =  { .preferred_node =  -1 },
> >  };
> >  
> 
> fairly clear.
> 
> >  static void mpol_rebind_policy(struct mempolicy *pol,
> > @@ -180,7 +184,8 @@ static struct mempolicy *mpol_new(int mo
> >  		 mode, nodes ? nodes_addr(*nodes)[0] : -1);
> >  
> >  	if (mode == MPOL_DEFAULT)
> > -		return NULL;
> > +		return NULL;	/* simply delete any existing policy */
> > +
> 
> Why do we not return default_policy and insert that into the VMA or
> whatever?
> 

Because then, if we're installing a shared policy [shmem], we'll go
ahead and create an rb-node and insert the [default] policy in the tree
in the shared policy struct, instead of just deleting any policy ranges
that the new policy covers.  Andi already implemented the code to delete
shared policy ranges covered by a subsequent null/default policy.  I
like this approach.

I have additional patches, to come later, that dynamically allocate the
shared policy structure when a non-null [non-default] policy is
installed.  At some point, I plan on enhancing this to to use a single
policy pointer, instead of the shared policy struct, when the policy
covers the entire object range, and delete any existing shared policy
struct when/if a default policy covers the entire range.

Thanks,
Lee

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