Messages in this thread | | | Subject | Re: [PATCH 2/8] mm: alloc_contig_freed_pages() added | Date | Wed, 21 Sep 2011 15:17:50 +0200 | From | "Michal Nazarewicz" <> |
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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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