On Thu, 06 Oct 2011 15:54:42 +0200
Marek Szyprowski [off-list ref] wrote:

From: KAMEZAWA Hiroyuki <redacted>

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