Subject: Re: [RFC PATCH] mm/init: fix zone boundary creation

From: Mel Gorman <[email-masked]>
Date: Mon, 30 May 2016 10:15:04 +0100

On Thu, May 26, 2016 at 02:21:42PM -0700, Andrew Morton wrote:
> On Thu,  5 May 2016 17:57:13 +1000 "Oliver O'Halloran" <[email-masked]> wrote:
> 
> > As a part of memory initialisation the architecture passes an array to
> > free_area_init_nodes() which specifies the max PFN of each memory zone.
> > This array is not necessarily monotonic (due to unused zones) so this
> > array is parsed to build monotonic lists of the min and max PFN for
> > each zone. ZONE_MOVABLE is special cased here as its limits are managed by
> > the mm subsystem rather than the architecture. Unfortunately, this special
> > casing is broken when ZONE_MOVABLE is the not the last zone in the zone
> > list. The core of the issue is:
> > 
> > 	if (i == ZONE_MOVABLE)
> > 		continue;
> > 	arch_zone_lowest_possible_pfn[i] =
> > 		arch_zone_highest_possible_pfn[i-1];
> > 
> > As ZONE_MOVABLE is skipped the lowest_possible_pfn of the next zone
> > will be set to zero. This patch fixes this bug by adding explicitly
> > tracking where the next zone should start rather than relying on the
> > contents arch_zone_highest_possible_pfn[].
> 
> hm, this is all ten year old Mel code.
> 

ZONE_MOVABLE at the time always existed at the end of a node during
initialisation time. It was allowed because the memory was always "stolen"
from the end of the node where it could have the same limitations as
ZONE_HIGHMEM if necessary. It was also safe to assume that zones never
overlapped as zones were about addressing limitations.  If ZONE_CMA or
ZONE_DEVICE can overlap with other zones during initialisation time then
there may be a few gremlins hiding in there. Unfortunately I have not
done an audit searching for problems with overlapping zones.

-- 
Mel Gorman
SUSE Labs