path: root/mm
diff options
authorMel Gorman <mel@csn.ul.ie>2009-05-13 17:34:48 +0100
committerRussell King <rmk+kernel@arm.linux.org.uk>2009-05-18 11:22:24 +0100
commiteb33575cf67d3f35fa2510210ef92631266e2465 (patch)
tree55dd9958dd10758aa5b1ad0186a3356ae620da44 /mm
parente1342f1da06d39b3bbd530e9306347c4438bc6e5 (diff)
[ARM] Double check memmap is actually valid with a memmap has unexpected holes V2
pfn_valid() is meant to be able to tell if a given PFN has valid memmap associated with it or not. In FLATMEM, it is expected that holes always have valid memmap as long as there is valid PFNs either side of the hole. In SPARSEMEM, it is assumed that a valid section has a memmap for the entire section. However, ARM and maybe other embedded architectures in the future free memmap backing holes to save memory on the assumption the memmap is never used. The page_zone linkages are then broken even though pfn_valid() returns true. A walker of the full memmap must then do this additional check to ensure the memmap they are looking at is sane by making sure the zone and PFN linkages are still valid. This is expensive, but walkers of the full memmap are extremely rare. This was caught before for FLATMEM and hacked around but it hits again for SPARSEMEM because the page_zone linkages can look ok where the PFN linkages are totally screwed. This looks like a hatchet job but the reality is that any clean solution would end up consumning all the memory saved by punching these unexpected holes in the memmap. For example, we tried marking the memmap within the section invalid but the section size exceeds the size of the hole in most cases so pfn_valid() starts returning false where valid memmap exists. Shrinking the size of the section would increase memory consumption offsetting the gains. This patch identifies when an architecture is punching unexpected holes in the memmap that the memory model cannot automatically detect and sets ARCH_HAS_HOLES_MEMORYMODEL. At the moment, this is restricted to EP93xx which is the model sub-architecture this has been reported on but may expand later. When set, walkers of the full memmap must call memmap_valid_within() for each PFN and passing in what it expects the page and zone to be for that PFN. If it finds the linkages to be broken, it assumes the memmap is invalid for that PFN. Signed-off-by: Mel Gorman <mel@csn.ul.ie> Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Diffstat (limited to 'mm')
2 files changed, 19 insertions, 15 deletions
diff --git a/mm/mmzone.c b/mm/mmzone.c
index 16ce8b955dc..f5b7d176021 100644
--- a/mm/mmzone.c
+++ b/mm/mmzone.c
@@ -6,6 +6,7 @@
#include <linux/stddef.h>
+#include <linux/mm.h>
#include <linux/mmzone.h>
#include <linux/module.h>
@@ -72,3 +73,17 @@ struct zoneref *next_zones_zonelist(struct zoneref *z,
*zone = zonelist_zone(z);
return z;
+int memmap_valid_within(unsigned long pfn,
+ struct page *page, struct zone *zone)
+ if (page_to_pfn(page) != pfn)
+ return 0;
+ if (page_zone(page) != zone)
+ return 0;
+ return 1;
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 66f6130976c..74d66dba0cb 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -509,22 +509,11 @@ static void pagetypeinfo_showblockcount_print(struct seq_file *m,
page = pfn_to_page(pfn);
- /*
- * Ordinarily, memory holes in flatmem still have a valid
- * memmap for the PFN range. However, an architecture for
- * embedded systems (e.g. ARM) can free up the memmap backing
- * holes to save memory on the assumption the memmap is
- * never used. The page_zone linkages are then broken even
- * though pfn_valid() returns true. Skip the page if the
- * linkages are broken. Even if this test passed, the impact
- * is that the counters for the movable type are off but
- * fragmentation monitoring is likely meaningless on small
- * systems.
- */
- if (page_zone(page) != zone)
+ /* Watch for unexpected holes punched in the memmap */
+ if (!memmap_valid_within(pfn, page, zone))
mtype = get_pageblock_migratetype(page);
if (mtype < MIGRATE_TYPES)