Re: [Linaro-mm-sig] [PATCH 2/9] mm: alloc_contig_freed_pages() added

On Thu, 06 Oct 2011 15:54:42 +0200 Marek Szyprowski m.szyprowski@samsung.com wrote:

From: KAMEZAWA Hiroyuki kamezawa.hiroyu@jp.fujitsu.com

This commit introduces alloc_contig_freed_pages() function

The "freed" seems redundant to me. Wouldn't "alloc_contig_pages" be a better name?

which allocates (ie. removes from buddy system) free pages in range. Caller has to guarantee that all pages in range are in buddy system.

Along with this function, a free_contig_pages() function is provided which frees all (or a subset of) pages allocated with alloc_contig_free_pages().

Michal Nazarewicz has modified the function to make it easier to allocate not MAX_ORDER_NR_PAGES aligned pages by making it return pfn of one-past-the-last allocated page.

...

+#if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP) +/*

• • Both PFNs must be from the same zone! If this function returns
• • true, pfn_to_page(pfn1) + (pfn2 - pfn1) == pfn_to_page(pfn2).
• */

+static inline bool zone_pfn_same_memmap(unsigned long pfn1, unsigned long pfn2) +{

• return pfn_to_section_nr(pfn1) == pfn_to_section_nr(pfn2);

+}

+#else

+#define zone_pfn_same_memmap(pfn1, pfn2) (true)

Do this in C, please. It's nicer and can prevent unused-var warnings.

+#endif

...

+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));
• page = pfn_to_page(start);
• zone = page_zone(page);
• spin_lock_irq(&zone->lock);
• for (;;) {

• VM_BUG_ON(page_count(page) || !PageBuddy(page) ||
  

• 	  page_zone(page) != zone);
  

• list_del(&page->lru);
  

• order = page_order(page);
  

• count = 1UL << order;
  

• zone->free_area[order].nr_free--;
  

• rmv_page_order(page);
  

• __mod_zone_page_state(zone, NR_FREE_PAGES, -(long)count);
  

__mod_zone_page_state() generally shouldn't be used - it bypasses the per-cpu magazines and can introduce high lock contentions.

That's hopefully not an issue on this callpath but it is still a red flag. I'd suggest at least the addition of a suitably apologetic code comment here - we don't want people to naively copy this code.

Plus such a comment would let me know why this was done ;)

• pfn += count;
  

• if (pfn >= end)
  

• 	break;
  

• VM_BUG_ON(!pfn_valid(pfn));
  

• if (zone_pfn_same_memmap(pfn - count, pfn))
  

• 	page += count;
  

• else
  

• 	page = pfn_to_page(pfn);
  

• }
• spin_unlock_irq(&zone->lock);
• /* After this, pages in the range can be freed one be one */
• count = pfn - start;
• pfn = start;
• for (page = pfn_to_page(pfn); count; --count) {

• prep_new_page(page, 0, flag);
  

• ++pfn;
  

• if (likely(zone_pfn_same_memmap(pfn - 1, pfn)))
  

• 	++page;
  

• else
  

• 	page = pfn_to_page(pfn);
  

• }
• return pfn;

+}

+void free_contig_pages(unsigned long pfn, unsigned nr_pages) +{

• struct page *page = pfn_to_page(pfn);
• while (nr_pages--) {

• __free_page(page);
  

• ++pfn;
  

• if (likely(zone_pfn_same_memmap(pfn - 1, pfn)))
  

• 	++page;
  

• else
  

• 	page = pfn_to_page(pfn);
  

• }

+}

You're sure these functions don't need EXPORT_SYMBOL()? Maybe the design is that only DMA core calls into here (if so, that's good).

#ifdef CONFIG_MEMORY_HOTREMOVE /*

• All pages in the range must be isolated before calling this.

-- 1.7.1.569.g6f426