linux-kernel - Re: [PATCH 2/8] mm: alloc_contig_freed

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Date:	Wed, 21 Sep 2011 15:17:50 +0200
From:	"Michal Nazarewicz" <mina86@...a86.com>
To:	"Marek Szyprowski" <m.szyprowski@...sung.com>,
	"Dave Hansen" <dave@...ux.vnet.ibm.com>
Cc:	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>,
	"Mel Gorman" <mel@....ul.ie>, "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 2/8] mm: alloc_contig_freed_pages() added

On Thu, 08 Sep 2011 20:05:52 +0200, Dave Hansen <dave@...ux.vnet.ibm.com>  
wrote:

> On Fri, 2011-08-19 at 16:27 +0200, Marek Szyprowski wrote:
>> +unsigned long alloc_contig_freed_pages(unsigned long start, unsigned  
>> long end,
>> +				       gfp_t flag)
>> +{
>> +	unsigned long pfn = start, count;
>> +	struct page *page;
>> +	struct zone *zone;
>> +	int order;
>> +
>> +	VM_BUG_ON(!pfn_valid(start));
>> +	zone = page_zone(pfn_to_page(start));
>
> This implies that start->end are entirely contained in a single zone.
> What enforces that?

In case of CMA, the __cma_activate_area() function from 6/8 has the check:

  151                         VM_BUG_ON(!pfn_valid(pfn));
  152                         VM_BUG_ON(page_zone(pfn_to_page(pfn)) !=  
zone);

This guarantees that CMA will never try to call alloc_contig_freed_pages()
on a region that spans multiple regions.

> If some higher layer enforces that, I think we probably need at least
> a VM_BUG_ON() in here and a comment about who enforces it.

Agreed.

>> +	spin_lock_irq(&zone->lock);
>> +
>> +	page = pfn_to_page(pfn);
>> +	for (;;) {
>> +		VM_BUG_ON(page_count(page) || !PageBuddy(page));
>> +		list_del(&page->lru);
>> +		order = page_order(page);
>> +		zone->free_area[order].nr_free--;
>> +		rmv_page_order(page);
>> +		__mod_zone_page_state(zone, NR_FREE_PAGES, -(1UL << order));
>> +		pfn  += 1 << order;
>> +		if (pfn >= end)
>> +			break;
>> +		VM_BUG_ON(!pfn_valid(pfn));
>> +		page += 1 << order;
>> +	}

> This 'struct page *'++ stuff is OK, but only for small, aligned areas.
> For at least some of the sparsemem modes (non-VMEMMAP), you could walk
> off of the end of the section_mem_map[] when you cross a MAX_ORDER
> boundary.  I'd feel a little bit more comfortable if pfn_to_page() was
> being done each time, or only occasionally when you cross a section
> boundary.

I'm fine with that.  I've used pointer arithmetic for performance reasons
but if that may potentially lead to bugs then obviously pfn_to_page()  
should
be used.

-- 
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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