memory hotplug: allocate usemap on the section with pgdat

Usemaps are allocated on the section which has pgdat by this.

Because usemap size is very small, many other sections usemaps are allocated
on only one page.  If a section has usemap, it can't be removed until removing
other sections.  This dependency is not desirable for memory removing.

Pgdat has similar feature.  When a section has pgdat area, it must be the last
section for removing on the node.  So, if section A has pgdat and section B
has usemap for section A, Both sections can't be removed due to dependency
each other.

To solve this issue, this patch collects usemap on same section with pgdat.
If other sections doesn't have any dependency, this section will be able to be
removed finally.

Signed-off-by: Yasunori Goto <y-goto@jp.fujitsu.com>
Cc: Badari Pulavarty <pbadari@us.ibm.com>
Cc: Yinghai Lu <yhlu.kernel@gmail.com>
Cc: Yasunori Goto <y-goto@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/sparse.c

@@ -249,11 +249,22 @@ static unsigned long *__kmalloc_section_usemap(void)
 
 static unsigned long *__init sparse_early_usemap_alloc(unsigned long pnum)
 {
-	unsigned long *usemap;
+	unsigned long *usemap, section_nr;
 	struct mem_section *ms = __nr_to_section(pnum);
 	int nid = sparse_early_nid(ms);
+	struct pglist_data *pgdat = NODE_DATA(nid);
 
-	usemap = alloc_bootmem_node(NODE_DATA(nid), usemap_size());
+	/*
+	 * Usemap's page can't be freed until freeing other sections
+	 * which use it. And, Pgdat has same feature.
+	 * If section A has pgdat and section B has usemap for other
+	 * sections (includes section A), both sections can't be removed,
+	 * because there is the dependency each other.
+	 * To solve above issue, this collects all usemap on the same section
+	 * which has pgdat.
+	 */
+	section_nr = pfn_to_section_nr(__pa(pgdat) >> PAGE_SHIFT);
+	usemap = alloc_bootmem_section(usemap_size(), section_nr);
 	if (usemap)
 		return usemap;