On Thu, 08 Sep 2011 20:05:52 +0200, Dave Hansen [off-list ref]  
wrote:

On Fri, 2011-08-19 at 16:27 +0200, Marek Szyprowski wrote:

quoted

+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.

quoted

+	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)
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