Re: [PATCH 6/6] Use one zonelist that is filtered by nodemask

[Nishanth Aravamudan]

On 08.10.2007 [18:56:05 -0700], Christoph Lameter wrote:
> On Mon, 8 Oct 2007, Nishanth Aravamudan wrote:
> 
> > >  struct page * fastcall
> > >  __alloc_pages(gfp_t gfp_mask, unsigned int order,
> > >  		struct zonelist *zonelist)
> > >  {
> > > +	/*
> > > +	 * Use a temporary nodemask for __GFP_THISNODE allocations. If the
> > > +	 * cost of allocating on the stack or the stack usage becomes
> > > +	 * noticable, allocate the nodemasks per node at boot or compile time
> > > +	 */
> > > +	if (unlikely(gfp_mask & __GFP_THISNODE)) {
> > > +		nodemask_t nodemask;
> > > +
> > > +		return __alloc_pages_internal(gfp_mask, order,
> > > +				zonelist, nodemask_thisnode(&nodemask));
> > > +	}
> > > +
> > >  	return __alloc_pages_internal(gfp_mask, order, zonelist, NULL);
> > >  }
> > 
> > <snip>
> > 
> > So alloc_pages_node() calls here and for THISNODE allocations, we go ask
> > nodemask_thisnode() for a nodemask...
> 
> Hmmmm... nodemask_thisnode needs to be passed the zonelist.
> 
> > And nodemask_thisnode() always gives us a nodemask with only the node
> > the current process is running on set, I think?
> 
> Right.
> 
> 
> > That seems really wrong -- and would explain what Lee was seeing while
> > using my patches for the hugetlb pool allocator to use THISNODE
> > allocations. All the allocations would end up coming from whatever node
> > the process happened to be running on. This obviously messes up hugetlb
> > accounting, as I rely on THISNODE requests returning NULL if they go
> > off-node.
> > 
> > I'm not sure how this would be fixed, as __alloc_pages() no longer has
> > the nid to set in the mask.
> > 
> > Am I wrong in my analysis?
> 
> No you are right on target. The thisnode function must determine the
> node from the first zone of the zonelist.

It seems like I would zonelist_node_idx() for this, along the lines of:

	static nodemask_t *nodemask_thisnode(nodemask_t *nodemask,
		struct zonelist *zonelist)
	{
		int nid = zonelist_node_idx(zonelist);
		/* Build a nodemask for just this node */
		nodes_clear(*nodemask);
		node_set(nid, *nodemask);

		return nodemask;
	}

But I think I need to check that zonelist->_zonerefs->zone is !NULL, given this
definition of zonelist_node_idx()

	static inline int zonelist_node_idx(struct zoneref *zoneref)
	{
	#ifdef CONFIG_NUMA
		/* zone_to_nid not available in this context */
		return zoneref->zone->node;
	#else
		return 0;
	#endif /* CONFIG_NUMA */
	}

and this comment in __alloc_pages_internal():

	....
	z = zonelist->_zonerefs;  /* the list of zones suitable for gfp_mask */

	if (unlikely(!z->zone)) {
		/*
		 * Happens if we have an empty zonelist as a result of
		 * GFP_THISNODE being used on a memoryless node
		 */
		return NULL;
	}
	...

It seems like zoneref->zone may be NULL in zonelist_node_idx()? Maybe
someone else should look into resolving this :)

Thanks,
Nish

-- 
Nishanth Aravamudan <nacc@xxxxxxxxxx>
IBM Linux Technology Center
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