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

From: Christoph Lameter
Date: Fri Jul 11 2014 - 11:59:16 EST

On Fri, 11 Jul 2014, Tejun Heo wrote:

> Hello,
> On Fri, Jul 11, 2014 at 10:13:57AM -0500, Christoph Lameter wrote:
> > Allocators typically fall back but they wont in some cases if you say
> > that you want memory from a particular node. A GFP_THISNODE would force a
> > failure of the alloc. In other cases it should fall back. I am not sure
> > that all allocations obey these conventions though.
> But, GFP_THISNODE + numa_mem_id() is identical to numa_node_id() +
> nearest node with memory fallback. Is there any case where the user
> would actually want to always fail if it's on the memless node?

GFP_THISNODE allocatios must fail if there is no memory available on
the node. No fallback allowed.

If the allocator performs caching for a particular node (like SLAB) then
the allocator *cannnot* accept memory from another node and the alloc via
the page allocator must fail so that the allocator can then pick another
node for keeping track of the allocations.

> Even if that's the case, there's no reason to burden everyone with
> this distinction. Most users just wanna say "I'm on this node.
> Please allocate considering that". There's nothing wrong with using
> numa_node_id() for that.

Well yes that speaks for this patch.

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