linux-kernel - Re: [PATCH 05/12] mm: alloc_contig

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Date:	Thu, 31 Mar 2011 23:17:51 +0200
From:	"Michal Nazarewicz" <mina86@...a86.com>
To:	"Dave Hansen" <dave@...ux.vnet.ibm.com>
Cc:	"Marek Szyprowski" <m.szyprowski@...sung.com>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-samsung-soc@...r.kernel.org, linux-media@...r.kernel.org,
	linux-mm@...ck.org, "Kyungmin Park" <kyungmin.park@...sung.com>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>,
	"Ankita Garg" <ankita@...ibm.com>,
	"Daniel Walker" <dwalker@...eaurora.org>,
	"Johan MOSSBERG" <johan.xx.mossberg@...ricsson.com>,
	"Mel Gorman" <mel@....ul.ie>, "Pawel Osciak" <pawel@...iak.com>
Subject: Re: [PATCH 05/12] mm: alloc_contig_range() added

On Thu, 31 Mar 2011 22:28:42 +0200, Dave Hansen <dave@...ux.vnet.ibm.com>  
wrote:

> On Thu, 2011-03-31 at 18:26 +0200, Michal Nazarewicz wrote:
>> > On Thu, 2011-03-31 at 15:16 +0200, Marek Szyprowski wrote:
>> >> +       ret = 0;
>> >> +       while (!PageBuddy(pfn_to_page(start & (~0UL << ret))))
>> >> +               if (WARN_ON(++ret >= MAX_ORDER))
>> >> +                       return -EINVAL;
>>
>> On Thu, 31 Mar 2011 18:02:41 +0200, Dave Hansen wrote:
>> > Holy cow, that's dense.  Is there really no more straightforward way  
>> to
>> > do that?
>>
>> Which part exactly is dense?  What would be qualify as a more
>> straightforward way?
>
> I'm still not 100% sure what it's trying to do.  It looks like it
> attempts to check all of "start"'s buddy pages.

No.  I'm going up through parents.  This is because even though start
falls in a free block (ie. one that page allocator tracks), the actual
page that is in buddy system is larger then start and this loop looks
for beginning of that page.

> int order;
> for (order = 0; order <= MAX_ORDER; order++) {
> 	unsigned long buddy_pfn = find_buddy(start, order);
> 	struct page *buddy = pfn_to_page(buddy_pfn);
> 	if (PageBuddy(buddy)
> 		break;
> 	WARN();
> 	return -EINVAL;
> }

The WARN() and return would have to be outside of the loop and, as I
described, instead of find_buddy() something like find_parent() would
have to be used.

> I'm wondering also if you can share some code with __rmqueue().

Doubtful since start does not (have to) point to a page that is tracked
by page allocator but a page inside such a page.

-- 
Best regards,                                         _     _
.o. | Liege of Serenely Enlightened Majesty of      o' \,=./ `o
..o | Computer Science,  Michal "mina86" Nazarewicz    (o o)
ooo +-----<email/xmpp: mnazarewicz@...gle.com>-----ooO--(_)--Ooo--
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