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SubjectRe: [PATCH 2/8] mm: alloc_contig_freed_pages() added
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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.

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


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