linux-kernel - Re: [PATCH 4/6] mm/vmstat: add zone range overlapping check

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Date:	Thu, 24 Mar 2016 09:08:31 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Mel Gorman <mgorman@...e.de>,
	Laura Abbott <lauraa@...eaurora.org>,
	Minchan Kim <minchan@...nel.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Michal Nazarewicz <mina86@...a86.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	"Rafael J. Wysocki" <rjw@...ysocki.net>
Subject: Re: [PATCH 4/6] mm/vmstat: add zone range overlapping check

On Wed, Mar 23, 2016 at 03:47:46PM +0100, Vlastimil Babka wrote:
> On 03/14/2016 08:31 AM, js1304@...il.com wrote:
> >From: Joonsoo Kim <iamjoonsoo.kim@....com>
> >
> >There is a system that node's pfn are overlapped like as following.
> >
> >-----pfn-------->
> >N0 N1 N2 N0 N1 N2
> >
> >Therefore, we need to care this overlapping when iterating pfn range.
> >
> >There are two places in vmstat.c that iterates pfn range and
> >they don't consider this overlapping. Add it.
> >
> >Without this patch, above system could over count pageblock number
> >on a zone.
> >
> >Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
> >---
> >  mm/vmstat.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> >
> >diff --git a/mm/vmstat.c b/mm/vmstat.c
> >index 5e43004..0a726e3 100644
> >--- a/mm/vmstat.c
> >+++ b/mm/vmstat.c
> >@@ -1010,6 +1010,9 @@ static void pagetypeinfo_showblockcount_print(struct seq_file *m,
> >  		if (!memmap_valid_within(pfn, page, zone))
> >  			continue;
> 
> The above already does this for each page within the block, but it's
> guarded by CONFIG_ARCH_HAS_HOLES_MEMORYMODEL. I guess that's not the
> case of your system, right?
> 
> I guess your added check should go above this, though. Also what
> about employing pageblock_pfn_to_page() here and in all other
> applicable places, so it's unified and optimized by
> zone->contiguous?

Comment on memmap_valid_within() in mmzone.h says that page_zone()
linkages could be broken in that system even if pfn_valid() returns
true. So, we cannot do zone check before it.

In fact, I wonder how that system works fine under the situation where
there are many pfn interators which doesn't check
memmap_valid_within(). I guess there may be enough constraint.
Anyway, I think that it is another issue and would be revisited later.

Thanks.