linux-kernel - Re: [RFC Patch V1 07/30] mm: Use cpu_to_mem()/numa_mem

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Date:	Fri, 11 Jul 2014 10:58:52 -0500 (CDT)
From:	Christoph Lameter <cl@...two.org>
To:	Tejun Heo <tj@...nel.org>
cc:	Jiang Liu <jiang.liu@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
	Vladimir Davydov <vdavydov@...allels.com>,
	Johannes Weiner <hannes@...xchg.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>,
	Wanpeng Li <liwanp@...ux.vnet.ibm.com>,
	Zhang Yanfei <zhangyanfei@...fujitsu.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Jianyu Zhan <nasa4836@...il.com>, malc <av1474@...tv.ru>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Fabian Frederick <fabf@...net.be>,
	Tony Luck <tony.luck@...el.com>, linux-mm@...ck.org,
	linux-hotplug@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC Patch V1 07/30] mm: Use cpu_to_mem()/numa_mem_id() to
 support memoryless node

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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