linux-kernel - Re: [RFC PATCH] mm: support CONFIG_ZONE_DEVICE + CONFIG_ZONE

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Date:	Tue, 26 Jan 2016 09:07:02 -0800
From:	Dan Williams <dan.j.williams@...el.com>
To:	Sudip Mukherjee <sudipm.mukherjee@...il.com>
Cc:	Mark <markk@...ra.co.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	"linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@....de>, Linux MM <linux-mm@...ck.org>,
	Ingo Molnar <mingo@...hat.com>, Mel Gorman <mgorman@...e.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Jerome Glisse <j.glisse@...il.com>
Subject: Re: [RFC PATCH] mm: support CONFIG_ZONE_DEVICE + CONFIG_ZONE_DMA

On Mon, Jan 25, 2016 at 10:00 PM, Sudip Mukherjee
<sudipm.mukherjee@...il.com> wrote:
> On Mon, Jan 25, 2016 at 04:06:40PM -0800, Dan Williams wrote:
>> It appears devices requiring ZONE_DMA are still prevalent (see link
>> below).  For this reason the proposal to require turning off ZONE_DMA to
>> enable ZONE_DEVICE is untenable in the short term.  We want a single
>> kernel image to be able to support legacy devices as well as next
>> generation persistent memory platforms.
>>
>> Towards this end, alias ZONE_DMA and ZONE_DEVICE to work around needing
>> to maintain a unique zone number for ZONE_DEVICE.  Record the geometry
>> of ZONE_DMA at init (->init_spanned_pages) and use that information in
>> is_zone_device_page() to differentiate pages allocated via
>> devm_memremap_pages() vs true ZONE_DMA pages.  Otherwise, use the
>> simpler definition of is_zone_device_page() when ZONE_DMA is turned off.
>>
>> Note that this also teaches the memory hot remove path that the zone may
>> not have sections for all pfn spans (->zone_dyn_start_pfn).
>>
>> A user visible implication of this change is potentially an unexpectedly
>> high "spanned" value in /proc/zoneinfo for the DMA zone.
>>
>> Cc: H. Peter Anvin <hpa@...or.com>
>> Cc: Ingo Molnar <mingo@...hat.com>
>> Cc: Rik van Riel <riel@...hat.com>
>> Cc: Mel Gorman <mgorman@...e.de>
>> Cc: Jerome Glisse <j.glisse@...il.com>
>> Cc: Christoph Hellwig <hch@....de>
>> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
>> Link: https://bugzilla.kernel.org/show_bug.cgi?id=110931
>> Fixes: 033fbae988fc ("mm: ZONE_DEVICE for "device memory"")
>> Reported-by: Sudip Mukherjee <sudipm.mukherjee@...il.com>
>
> It should actually be Reported-by: Mark <markk@...ra.co.uk>
>
> Hi Mark,
> Can you please test this patch available at https://patchwork.kernel.org/patch/8116991/
> in your setup..

Note this patch is on top of 4.5-rc1 and is likely not a suitable for
-stable backport to 4.3/4.4.  For 4.3 and 4.4, distributions that want
to support legacy devices should leave ZONE_DEVICE disabled as it is
by default.