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

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Date:	Thu, 08 Sep 2011 11:05:52 -0700
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Marek Szyprowski <m.szyprowski@...sung.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,
	Michal Nazarewicz <mina86@...a86.com>,
	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 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?  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.

> +	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.

This may not apply to what ARM is doing today, but it shouldn't be too
difficult to fix up, or to document what's going on.

> +	spin_unlock_irq(&zone->lock);
> +
> +	/* After this, pages in the range can be freed one be one */
> +	page = pfn_to_page(start);
> +	for (count = pfn - start; count; --count, ++page)
> +		prep_new_page(page, 0, flag);
> +
> +	return pfn;
> +}
> +
> +void free_contig_pages(struct page *page, int nr_pages)
> +{
> +	for (; nr_pages; --nr_pages, ++page)
> +		__free_page(page);
> +}

The same thing about 'struct page' pointer math goes here.

-- Dave

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