On Thu, 08 Sep 2011 20:05:52 +0200, Dave Hansen dave@linux.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.