linux-kernel - Re: [PATCH v2] mm: exclude ZONE_DEVICE from GFP_ZONE

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Date:	Fri, 4 Mar 2016 07:59:48 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux MM <linux-mm@...ck.org>, Mel Gorman <mgorman@...e.de>,
	Mark <markk@...ra.co.uk>, Joonsoo Kim <iamjoonsoo.kim@....com>,
	Sudip Mukherjee <sudipm.mukherjee@...il.com>
Subject: Re: [PATCH v2] mm: exclude ZONE_DEVICE from GFP_ZONE_TABLE

On Fri, Mar 4, 2016 at 6:11 AM, Vlastimil Babka <vbabka@...e.cz> wrote:
> On 03/02/2016 01:32 AM, Dan Williams wrote:
>> ZONE_DEVICE (merged in 4.3) and ZONE_CMA (proposed) are examples of new
>> mm zones that are bumping up against the current maximum limit of 4
>> zones, i.e. 2 bits in page->flags for the GFP_ZONE_TABLE.
>>
>> The GFP_ZONE_TABLE poses an interesting constraint since
>> include/linux/gfp.h gets included by the 32-bit portion of a 64-bit
>> build.  We need to be careful to only build the table for zones that
>> have a corresponding gfp_t flag.  GFP_ZONES_SHIFT is introduced for this
>> purpose.  This patch does not attempt to solve the problem of adding a
>> new zone that also has a corresponding GFP_ flag.
>>
>> Vlastimil points out that ZONE_DEVICE, by depending on x86_64 and
>> SPARSEMEM_VMEMMAP implies that SECTIONS_WIDTH is zero.  In other words
>
>                                                        ^ by default
>
> Because CONFIG_SPARSEMEM_VMEMMAP can still be disabled by the user.
>
>> even though ZONE_DEVICE does not fit in GFP_ZONE_TABLE it is free to
>> consume another bit in page->flags (expand ZONES_WIDTH) with room to
>> spare.
>
> So it's still possible to configure the x86_64 kernel such that you get
> "#warning Unfortunate NUMA and NUMA Balancing config". But it requires
> some effort to override the defaults, and it's not breaking build or
> runtime. BTW I was able to get that warning even with your previous
> patch that limited NODES_WIDTH, so that wasn't a solution for this
> anyway. This patch is simpler and better.

All this suggests that ZONE_DEVICE depend on SPARSEMEM_VMEMMAP.  I'll
fix that up.

>> Link: https://bugzilla.kernel.org/show_bug.cgi?id=110931
>> Fixes: 033fbae988fc ("mm: ZONE_DEVICE for "device memory"")
>> Cc: Mel Gorman <mgorman@...e.de>
>> Cc: Rik van Riel <riel@...hat.com>
>> Cc: Joonsoo Kim <iamjoonsoo.kim@....com>
>> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
>> Cc: Sudip Mukherjee <sudipm.mukherjee@...il.com>
>> Reported-by: Mark <markk@...ra.co.uk>
>> Reported-by: Vlastimil Babka <vbabka@...e.cz>
>> Signed-off-by: Dan Williams <dan.j.williams@...el.com>
>
> Acked-by: Vlastimil Babka <vbabka@...e.cz>

Thanks!