[PATCH] Simplify node/zone field in page->flags

Matthew Dobson
2003-12-22T19:56:38Z

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Currently we keep track of a pages node & zone in the top 8 bits (on
32-bit arches, 10 bits on 64-bit arches) of page->flags. We typically
compute the field as follows:
node_num * MAX_NR_ZONES + zone_num = 'nodezone'

It's non-trivial to break this 'nodezone' back into node and zone
numbers. This patch modifies the way we compute the index to be:
(node_num << ZONE_SHIFT) | zone_num

This makes it trivial to recover either the node or zone number with a
simple bitshift. There are many places in the kernel where we do things
like: page_zone(page)->zone_pgdat->node_id to determine the node a page
belongs to. With this patch we save several pointer dereferences, and
it all boils down to shifting some bits.

Cheers!

-Matt

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diff -Nurp --exclude-from=/home/mcd/.dontdiff linux-2.6.0-vanilla/include/linux/mm.h linux-2.6.0-patched/include/linux/mm.h
--- linux-2.6.0-vanilla/include/linux/mm.h Wed Dec 17 18:58:05 2003
+++ linux-2.6.0-patched/include/linux/mm.h Thu Dec 18 14:27:26 2003
@@ -323,20 +323,31 @@ static inline void put_page(struct page
* The zone field is never updated after free_area_init_core()
* sets it, so none of the operations on it need to be atomic.
*/
-#define ZONE_SHIFT (BITS_PER_LONG - 8)
+#define NODEZONE_SHIFT (BITS_PER_LONG - MAX_NODES_SHIFT - MAX_ZONES_SHIFT)
+#define NODEZONE(node, zone) ((node << ZONES_SHIFT) | zone)
+
+static inline unsigned long page_zonenum(struct page *page)
+{
+ return (page->flags >> NODEZONE_SHIFT) & (~(~0UL << ZONES_SHIFT));
+}
+
+static inline unsigned long page_nodenum(struct page *page)
+{
+ return (page->flags >> NODEZONE_SHIFT + ZONES_SHIFT);
+}

struct zone;
extern struct zone *zone_table[];

static inline struct zone *page_zone(struct page *page)
{
- return zone_table[page->flags >> ZONE_SHIFT];
+ return zone_table[page->flags >> NODEZONE_SHIFT];
}

-static inline void set_page_zone(struct page *page, unsigned long zone_num)
+static inline void set_page_zone(struct page *page, unsigned long nodezone_num)
{
- page->flags &= ~(~0UL << ZONE_SHIFT);
- page->flags |= zone_num << ZONE_SHIFT;
+ page->flags &= ~(~0UL << NODEZONE_SHIFT);
+ page->flags |= nodezone_num << NODEZONE_SHIFT;
}

#ifndef CONFIG_DISCONTIGMEM
diff -Nurp --exclude-from=/home/mcd/.dontdiff linux-2.6.0-vanilla/include/linux/mmzone.h linux-2.6.0-patched/include/linux/mmzone.h
--- linux-2.6.0-vanilla/include/linux/mmzone.h Wed Dec 17 18:58:57 2003
+++ linux-2.6.0-patched/include/linux/mmzone.h Thu Dec 18 14:27:26 2003
@@ -159,8 +159,10 @@ struct zone {
#define ZONE_DMA 0
#define ZONE_NORMAL 1
#define ZONE_HIGHMEM 2
-#define MAX_NR_ZONES 3
-#define GFP_ZONEMASK 0x03
+
+#define MAX_NR_ZONES 3 /* Sync this with ZONES_SHIFT */
+#define ZONES_SHIFT 2 /* = ceil(log2(MAX_NR_ZONES)) */
+#define GFP_ZONEMASK 0x03

/*
* One allocation request operates on a zonelist. A zonelist
@@ -310,7 +312,7 @@ extern struct pglist_data contig_page_da

#if BITS_PER_LONG == 32
/*
- * with 32 bit flags field, page->zone is currently 8 bits.
+ * with 32 bit page->flags field, we reserve 8 bits for node/zone info.
* there are 3 zones (2 bits) and this leaves 8-2=6 bits for nodes.
*/
#define MAX_NODES_SHIFT 6
@@ -327,6 +329,13 @@ extern struct pglist_data contig_page_da
#error NODES_SHIFT > MAX_NODES_SHIFT
#endif

+/* There are currently 3 zones: DMA, Normal & Highmem, thus we need 2 bits */
+#define MAX_ZONES_SHIFT 2
+
+#if ZONES_SHIFT > MAX_ZONES_SHIFT
+#error ZONES_SHIFT > MAX_ZONES_SHIFT
+#endif
+
extern DECLARE_BITMAP(node_online_map, MAX_NUMNODES);
extern DECLARE_BITMAP(memblk_online_map, MAX_NR_MEMBLKS);

diff -Nurp --exclude-from=/home/mcd/.dontdiff linux-2.6.0-vanilla/mm/page_alloc.c linux-2.6.0-patched/mm/page_alloc.c
--- linux-2.6.0-vanilla/mm/page_alloc.c Wed Dec 17 18:58:08 2003
+++ linux-2.6.0-patched/mm/page_alloc.c Thu Dec 18 14:27:26 2003
@@ -50,7 +50,7 @@ EXPORT_SYMBOL(nr_swap_pages);
* Used by page_zone() to look up the address of the struct zone whose
* id is encoded in the upper bits of page->flags
*/
-struct zone *zone_table[MAX_NR_ZONES*MAX_NUMNODES];
+struct zone *zone_table[1 << (ZONES_SHIFT + NODES_SHIFT)];
EXPORT_SYMBOL(zone_table);

static char *zone_names[MAX_NR_ZONES] = { "DMA", "Normal", "HighMem" };
@@ -1210,7 +1210,7 @@ void __init memmap_init_zone(struct page
struct page *page;

for (page = start; page < (start + size); page++) {
- set_page_zone(page, nid * MAX_NR_ZONES + zone);
+ set_page_zone(page, NODEZONE(nid, zone));
set_page_count(page, 0);
SetPageReserved(page);
INIT_LIST_HEAD(&page->list);
@@ -1251,7 +1251,7 @@ static void __init free_area_init_core(s
unsigned long size, realsize;
unsigned long batch;

- zone_table[nid * MAX_NR_ZONES + j] = zone;
+ zone_table[NODEZONE(nid, j)] = zone;
realsize = size = zones_size[j];
if (zholes_size)
realsize -= zholes_size[j];