[RFC PATCH v2 04/15] mm: Add helpers to retrieve node region and zone region for a given page

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

Given a page, we would like to have an efficient mechanism to find out
the node memory region and the zone memory region to which it belongs.

Since the node is assumed to be divided into equal-sized node memory
regions, the node memory region can be obtained by simply right-shifting
the page's pfn by 'MEM_REGION_SHIFT'.

But finding the corresponding zone memory region's index in the zone is
not that straight-forward. To have a O(1) algorithm to find it out, define a
zone_region_idx[] array to store the zone memory region indices for every
node memory region.

To illustrate, consider the following example:

	|<----------------------Node---------------------->|
	 __________________________________________________
	|      Node mem reg 0 	 |      Node mem reg 1     |  (Absolute region
	|________________________|_________________________|   boundaries)

	 __________________________________________________
	|    ZONE_DMA   |	    ZONE_NORMAL		   |
	|               |                                  |
	|<--- ZMR 0 --->|<-ZMR0->|<-------- ZMR 1 -------->|
	|_______________|________|_________________________|


In the above figure,

Node mem region 0:
------------------
This region corresponds to the first zone mem region in ZONE_DMA and also
the first zone mem region in ZONE_NORMAL. Hence its index array would look
like this:
    node_regions[0].zone_region_idx[ZONE_DMA]     == 0
    node_regions[0].zone_region_idx[ZONE_NORMAL]  == 0


Node mem region 1:
------------------
This region corresponds to the second zone mem region in ZONE_NORMAL. Hence
its index array would look like this:
    node_regions[1].zone_region_idx[ZONE_NORMAL]  == 1


Using this index array, we can quickly obtain the zone memory region to
which a given page belongs.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 include/linux/mm.h     |   24 ++++++++++++++++++++++++
 include/linux/mmzone.h |    7 +++++++
 mm/page_alloc.c        |    1 +
 3 files changed, 32 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index b7b368a..dff478b 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -717,6 +717,30 @@ static inline struct zone *page_zone(const struct page *page)
 	return &NODE_DATA(page_to_nid(page))->node_zones[page_zonenum(page)];
 }
 
+static inline int page_node_region_id(const struct page *page,
+				      const pg_data_t *pgdat)
+{
+	return (page_to_pfn(page) - pgdat->node_start_pfn) >> MEM_REGION_SHIFT;
+}
+
+/**
+ * Return the index of the zone memory region to which the page belongs.
+ *
+ * Given a page, find the absolute (node) memory region as well as the zone to
+ * which it belongs. Then find the region within the zone that corresponds to
+ * that node memory region, and return its index.
+ */
+static inline int page_zone_region_id(const struct page *page)
+{
+	pg_data_t *pgdat = NODE_DATA(page_to_nid(page));
+	enum zone_type z_num = page_zonenum(page);
+	unsigned long node_region_idx;
+
+	node_region_idx = page_node_region_id(page, pgdat);
+
+	return pgdat->node_regions[node_region_idx].zone_region_idx[z_num];
+}
+
 #ifdef SECTION_IN_PAGE_FLAGS
 static inline void set_page_section(struct page *page, unsigned long section)
 {
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 46a6b63..f772e05 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -703,6 +703,13 @@ struct node_mem_region {
 	unsigned long end_pfn;
 	unsigned long present_pages;
 	unsigned long spanned_pages;
+
+	/*
+	 * A physical (node) region could be split across multiple zones.
+	 * Store the indices of the corresponding regions of each such
+	 * zone for this physical (node) region.
+	 */
+	int zone_region_idx[MAX_NR_ZONES];
 	struct pglist_data *pgdat;
 };
 
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index d4abba6..af87471 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4749,6 +4749,7 @@ static void __meminit init_zone_memory_regions(struct pglist_data *pgdat)
 			zone_region->present_pages =
 					zone_region->spanned_pages - absent;
 
+			node_region->zone_region_idx[zone_idx(z)] = idx;
 			idx++;
 		}
 

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