Re: [RFC Patch V1 07/30] mm: Use cpu_to_mem()/numa_mem_id() to support memoryless node

[Christoph Lameter]

On Fri, 11 Jul 2014, Tejun Heo wrote:

> On Fri, Jul 11, 2014 at 10:55:59AM -0500, Christoph Lameter wrote:
> > > Where X is the memless node.  num_mem_id() on X would return either B
> > > or C, right?  If B or C can't satisfy the allocation, the allocator
> > > would fallback to A from B and D for C, both of which aren't optimal.
> > > It should first fall back to C or B respectively, which the allocator
> > > can't do anymoe because the information is lost when the caller side
> > > performs numa_mem_id().
> >
> > True but the advantage is that the numa_mem_id() allows the use of a
> > consitent sort of "local" node which increases allocator performance due
> > to the abillity to cache objects from that node.
>
> But the allocator can do the mapping the same.  I really don't see why
> we'd push the distinction to the individual users.

The "users" (I guess you mean general kernel code/drivers) can use various
memory allocators which will do the right thing internally regarding
GFP_THISNODE. They do not need to worry too much about this unless there
are reasons beyond optimizing NUMA placement to need memory from a
particuylar node (f.e. a device that requires memory from a numa node that
is local to the PCI bus where the hardware resides).

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