Re: [PATCH v2 3/7] x86, gfp: Cache best near node for memory allocation.

[Tang Chen]

Hi, tj,

On 09/27/2015 01:53 AM, Tejun Heo wrote:
> Hello, Tang.
>
> On Sat, Sep 26, 2015 at 05:31:07PM +0800, Tang Chen wrote:
> > > > @@ -307,13 +307,19 @@ static inline struct page *alloc_pages_node(int nid, gfp_t gfp_mask,
> > > >   	if (nid < 0)
> > > >   		nid = numa_node_id();
> > > > +	if (!node_online(nid))
> > > > +		nid = get_near_online_node(nid);
> > > > +
> > > >   	return __alloc_pages(gfp_mask, order, node_zonelist(nid, gfp_mask));
> > > >   }
> > > Why not just update node_data[]->node_zonelist in the first place?
> > zonelist will be rebuilt in __offline_pages() when the zone is not populated
> > any more.
> >
> > Here, getting the best near online node is for those cpus on memory-less
> > nodes.
> >
> > In the original code, if nid is NUMA_NO_NODE, the node the current cpu
> > resides in
> > will be chosen. And if the node is memory-less node, the cpu will be mapped
> > to its
> > best near online node.
> >
> > But this patch-set will map the cpu to its original node, so numa_node_id()
> > may return
> > a memory-less node to allocator. And then memory allocation may fail.
> Correct me if I'm wrong but the zonelist dictates which memory areas
> the page allocator is gonna try to from, right?  What I'm wondering is
> why we aren't handling memory-less nodes by simply updating their
> zonelists.  I mean, if, say, node 2 is memory-less, its zonelist can
> simply point to zones from other nodes, right?  What am I missing
> here?

Oh, yes, you are right. But I remember some time ago, Liu, Jiang has or was
going to handle memory less node like this in his patch:

https://lkml.org/lkml/2015/8/16/130

BTW, to Liu Jiang, how is your patches going on ?

Thanks.

> Thanks.

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