Below is a small extension of the numa file in the vm Documenation branch
which tries to give a brief explanation about pg_data_t and zone_t
structs. Patch is against 2.4.19pre7 but I think it'll apply to any 2.4.x
or 2.5.x kernel. No change of code etc etc etc. Comments, corrections and
opinions very welcome
Mel
--- linux-2.4.19pre7.orig/Documentation/vm/numa Fri Aug 4 19:23:37 2000
+++ linux-2.4.19pre7.mel/Documentation/vm/numa Fri Apr 19 04:46:23 2002
@@ -1,4 +1,5 @@
Started Nov 1999 by Kanoj Sarcar <kanoj@sgi.com>
+Appended Apr 2002 by Mel Gorman <melcsn.ul.ie>
The intent of this file is to have an uptodate, running commentary
from different people about NUMA specific code in the Linux vm.
@@ -39,3 +40,123 @@
NUMA port achieves more maturity. The call alloc_pages_node has been
added, so that drivers can make the call and not worry about whether
it is running on a NUMA or UMA platform.
+
+
+Nodes
+=====
+
+A node is described by the pg\_data\_t struct. Each can have one or more
+of the three zone types ZONE\_HIGHMEM, ZONE\_NORMAL and ZONE\_DMA. It can
+only have one zone of each type. It is the responsibility of the buddy
+allocator to make sure pages are allocated from the proper nodes.
+
+It is declared as
+
+typedef struct pglist_data {
+ zone_t node_zones[MAX_NR_ZONES];
+ zonelist_t node_zonelists[GFP_ZONEMASK+1];
+ int nr_zones;
+ struct page *node_mem_map;
+ unsigned long *valid_addr_bitmap;
+ struct bootmem_data *bdata;
+ unsigned long node_start_paddr;
+ unsigned long node_start_mapnr;
+ unsigned long node_size;
+ int node_id;
+ struct pglist_data *node_next;
+} pg_data_t;
+
+ node_zones The zones for this node. Currently ZONE_HIGHMEM,
+ ZONE_NORMAL, ZONE_DMA.
+
+ node_zonelists This is the order of zones that allocations are
+ preferred from. build_zonelists() in page_alloc.c does
+ the work when called by free_area_init_core(). So a failed
+ allocation ZONE_HIGHMEM may fall back to ZONE_NORMAL
+ or back to ZONE_DMA . See the buddy algorithm for details.
+
+ nr_zones Number of zones in this node, between 1 and 3
+
+ node_mem_map The first page of the physical block this node represents
+
+ valid_addr_bitmap Not positive, a bitmap that shows where holes are in memory?
+
+ bdata Used only when starting up the node. Mainly confined
+ to bootmem.c
+
+ node_start_paddr The starting physical addres of the node?
+
+ node_start_mapnr This appears to be a "nice" place to put the zone inside
+ the larger mem_map. It's set during
+ free_page_init_core. Presumably there is some architecture
+ dependant way of defining nice.
+
+ node_size The total number of pages in this zone
+
+ node_id The ID of the node, starts at 0
+
+ node_next Pointer to next node in a linear list. NULL terminated
+
+1.2 Zones
+===========
+ Each pg_data_t node will be aware of one or more zones that it can
+allocate pages from. The possible zones are ZONE_HIGHMEM, ZONE_NORMAL
+and ZONE_DMA. There can only be one zone of each type per pg_data_t.
+Each zone is suitable for a particular use but there is not necessarily
+a penalty for usage of the wrong zone like there is with the wrong
+pg_data_t
+
+ typedef struct zone_struct {
+ /*
+ * Commonly accessed fields:
+ */
+ spinlock_t lock;
+ unsigned long free_pages;
+ unsigned long pages_min, pages_low, pages_high;
+ int need_balance;
+
+ /*
+ * free areas of different sizes
+ */
+ free_area_t free_area[MAX_ORDER];
+
+ /*
+ * Discontig memory support fields.
+ */
+ struct pglist_data *zone_pgdat;
+ struct page *zone_mem_map;
+ unsigned long zone_start_paddr;
+ unsigned long zone_start_mapnr;
+
+ /*
+ * rarely used fields:
+ */
+ char *name;
+ unsigned long size;
+ } zone_t;
+
+ lock A lock to protect the zone
+ free_pages Total number of free pages in the zone
+ pages_min When pages_min is reached, kswapd is woken up
+ pages_low When reached, the allocator will do the kswapd work in
+ a synchronuous fashion
+ pages_high Once kswapd is woken, it won't sleep until pages_high pages
+ are free
+ need_balance A flag kswapd uses to determine if it needs to balance
+ free_area Used by the buddy algorithm
+ zone_pgdat Points to the parent pg_data_t
+ zone_mem_map The first page in mem_map this zone refers to
+ zone_start_paddr Physical address of zone
+ zone_start_mapnr Address inside mem_map ?
+ name The string name of the zone
+ size Self explanatory
+
+1.3 Relationship
+==================
+
+ pg_data_t -------> pg_data_t -------> pgdata_t ------->NULL
+ / | \ / | \ / | \
+ ----- | ----- ----- | ----- ----- | -----
+ | | | | | | | | |
+ zone_t zone_t zone_t zone_t zone_t zone_t zone_t zone_t zone_t
+
-
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