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

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Date:	Fri, 4 Mar 2016 15:11:50 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Dan Williams <dan.j.williams@...el.com>, akpm@...ux-foundation.org
Cc:	Rik van Riel <riel@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	linux-kernel@...r.kernel.org, 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 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.

> 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>