linux-kernel - Re: [PATCH 03/11] mm: mmzone: introduce zone_pfn_same

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Date:	Mon, 12 Dec 2011 15:51:43 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Michal Nazarewicz <mina86@...a86.com>
Cc:	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-media@...r.kernel.org, linux-mm@...ck.org,
	linaro-mm-sig@...ts.linaro.org,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Russell King <linux@....linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Ankita Garg <ankita@...ibm.com>,
	Daniel Walker <dwalker@...eaurora.org>,
	Arnd Bergmann <arnd@...db.de>,
	Jesse Barker <jesse.barker@...aro.org>,
	Jonathan Corbet <corbet@....net>,
	Shariq Hasnain <shariq.hasnain@...aro.org>,
	Chunsang Jeong <chunsang.jeong@...aro.org>
Subject: Re: [PATCH 03/11] mm: mmzone: introduce zone_pfn_same_memmap()

On Mon, Dec 12, 2011 at 03:51:55PM +0100, Michal Nazarewicz wrote:
> >On Fri, Nov 18, 2011 at 05:43:10PM +0100, Marek Szyprowski wrote:
> >>From: Michal Nazarewicz <mina86@...a86.com>
> >>diff --git a/mm/compaction.c b/mm/compaction.c
> >>index 6afae0e..09c9702 100644
> >>--- a/mm/compaction.c
> >>+++ b/mm/compaction.c
> >>@@ -111,7 +111,10 @@ skip:
> >>
> >> next:
> >> 		pfn += isolated;
> >>-		page += isolated;
> >>+		if (zone_pfn_same_memmap(pfn - isolated, pfn))
> >>+			page += isolated;
> >>+		else
> >>+			page = pfn_to_page(pfn);
> >> 	}
> 
> On Mon, 12 Dec 2011 15:19:53 +0100, Mel Gorman <mel@....ul.ie> wrote:
> >Is this necessary?
> >
> >We are isolating pages, the largest of which is a MAX_ORDER_NR_PAGES
> >page.  [...]
> 
> On Mon, 12 Dec 2011 15:40:30 +0100, Mel Gorman <mel@....ul.ie> wrote:
> >To be clear, I'm referring to a single page being isolated here. It may
> >or may not be a high-order page but it's still going to be less then
> >MAX_ORDER_NR_PAGES so you should be able check when a new block is
> >entered and pfn_to_page is necessary.
> 
> Do you mean something like:
> 
> if (same pageblock)
> 	just do arithmetic;
> else
> 	use pfn_to_page;
> 

something like the following untested snippet.

/*
 * Resolve pfn_to_page every MAX_ORDER_NR_PAGES to handle the case where
 * memmap is not contiguous such as with SPARSEMEM memory model without
 * VMEMMAP
 */
pfn += isolated;
page += isolated;
if ((pfn & ~(MAX_ORDER_NR_PAGES-1)) == 0)
	page = pfn_to_page(pfn);

That would be closer to what other PFN walkers do

> ?
> 
> I've discussed it with Dave and he suggested that approach as an
> optimisation since in some configurations zone_pfn_same_memmap()
> is always true thus compiler will strip the else part, whereas
> same pageblock test will be false on occasions regardless of kernel
> configuration.
> 

Ok, while I recognise it's an optimisation, it's a very small
optimisation and I'm not keen on introducing something new for
CMA that has been coped with in the past by always walking PFNs in
pageblock-sized ranges with pfn_valid checks where necessary.

See setup_zone_migrate_reserve as one example where pfn_to_page is
only called once per pageblock and calls pageblock_is_reserved()
for examining pages within a pageblock. Still, if you really want
the helper, at least keep it in compaction.c as there should be no
need to have it in mmzone.h

-- 
Mel Gorman
SUSE Labs
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