mm: introduce new field "managed_pages" to struct zone
Currently a zone's present_pages is calcuated as below, which is
inaccurate and may cause trouble to memory hotplug.
spanned_pages - absent_pages - memmap_pages - dma_reserve.
During fixing bugs caused by inaccurate zone->present_pages, we found
zone->present_pages has been abused. The field zone->present_pages may
have different meanings in different contexts:
1) pages existing in a zone.
2) pages managed by the buddy system.
For more discussions about the issue, please refer to:
http://lkml.org/lkml/2012/11/5/866
https://patchwork.kernel.org/patch/1346751/
This patchset tries to introduce a new field named "managed_pages" to
struct zone, which counts "pages managed by the buddy system". And revert
zone->present_pages to count "physical pages existing in a zone", which
also keep in consistence with pgdat->node_present_pages.
We will set an initial value for zone->managed_pages in function
free_area_init_core() and will adjust it later if the initial value is
inaccurate.
For DMA/normal zones, the initial value is set to:
(spanned_pages - absent_pages - memmap_pages - dma_reserve)
Later zone->managed_pages will be adjusted to the accurate value when the
bootmem allocator frees all free pages to the buddy system in function
free_all_bootmem_node() and free_all_bootmem().
The bootmem allocator doesn't touch highmem pages, so highmem zones'
managed_pages is set to the accurate value "spanned_pages - absent_pages"
in function free_area_init_core() and won't be updated anymore.
This patch also adds a new field "managed_pages" to /proc/zoneinfo
and sysrq showmem.
[akpm@linux-foundation.org: small comment tweaks]
Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Cc: Wen Congyang <wency@cn.fujitsu.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Maciej Rutecki <maciej.rutecki@gmail.com>
Tested-by: Chris Clayton <chris2553@googlemail.com>
Cc: "Rafael J . Wysocki" <rjw@sisk.pl>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Minchan Kim <minchan@kernel.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Jianguo Wu <wujianguo@huawei.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 0c0b1d6..cd55dad 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -460,17 +460,44 @@
unsigned long zone_start_pfn;
/*
- * zone_start_pfn, spanned_pages and present_pages are all
- * protected by span_seqlock. It is a seqlock because it has
- * to be read outside of zone->lock, and it is done in the main
- * allocator path. But, it is written quite infrequently.
+ * spanned_pages is the total pages spanned by the zone, including
+ * holes, which is calculated as:
+ * spanned_pages = zone_end_pfn - zone_start_pfn;
*
- * The lock is declared along with zone->lock because it is
+ * present_pages is physical pages existing within the zone, which
+ * is calculated as:
+ * present_pages = spanned_pages - absent_pages(pags in holes);
+ *
+ * managed_pages is present pages managed by the buddy system, which
+ * is calculated as (reserved_pages includes pages allocated by the
+ * bootmem allocator):
+ * managed_pages = present_pages - reserved_pages;
+ *
+ * So present_pages may be used by memory hotplug or memory power
+ * management logic to figure out unmanaged pages by checking
+ * (present_pages - managed_pages). And managed_pages should be used
+ * by page allocator and vm scanner to calculate all kinds of watermarks
+ * and thresholds.
+ *
+ * Locking rules:
+ *
+ * zone_start_pfn and spanned_pages are protected by span_seqlock.
+ * It is a seqlock because it has to be read outside of zone->lock,
+ * and it is done in the main allocator path. But, it is written
+ * quite infrequently.
+ *
+ * The span_seq lock is declared along with zone->lock because it is
* frequently read in proximity to zone->lock. It's good to
* give them a chance of being in the same cacheline.
+ *
+ * Write access to present_pages and managed_pages at runtime should
+ * be protected by lock_memory_hotplug()/unlock_memory_hotplug().
+ * Any reader who can't tolerant drift of present_pages and
+ * managed_pages should hold memory hotplug lock to get a stable value.
*/
- unsigned long spanned_pages; /* total size, including holes */
- unsigned long present_pages; /* amount of memory (excluding holes) */
+ unsigned long spanned_pages;
+ unsigned long present_pages;
+ unsigned long managed_pages;
/*
* rarely used fields: