From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christoph Lameter <cl@gentwo.org>
Date: Fri, 11 Jul 2014 15:58:52 +0000
Subject: Re: [RFC Patch V1 07/30] mm: Use cpu_to_mem()/numa_mem_id() to support memoryless node
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To: Tejun Heo <tj@kernel.org>
Cc: Jiang Liu <jiang.liu@linux.intel.com>, Andrew Morton <akpm@linux-foundation.org>, Mel Gorman <mgorman@suse.de>, David Rientjes <rientjes@google.com>, Mike Galbraith <umgwanakikbuti@gmail.com>, Peter Zijlstra <peterz@infradead.org>, "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>, Vladimir Davydov <vdavydov@parallels.com>, Johannes Weiner <hannes@cmpxchg.org>, "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, Rik van Riel <riel@redhat.com>, Wanpeng Li <liwanp@linux.vnet.ibm.com>, Zhang Yanfei <zhangyanfei@cn.fujitsu.com>, Catalin Marinas <catalin.marinas@arm.com>, Jianyu Zhan <nasa4836@gmail.com>, malc <av1474@comtv.ru>, Joonsoo Kim <iamjoonsoo.kim@lge.com>, Fabian Frederick <fabf@skynet.be>, Tony Luck <tony.luck@intel.com>, linux-mm@kvack.org, linux-hotplug@vger.kernel.org, linux-kernel@vger.kernel.org

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.