[patch 1/4] mm: exclude reserved pages from dirtyable memory

Johannes Weiner
2011-09-20T13:46:13Z

The amount of dirtyable pages should not include the total number of
free pages: there is a number of reserved pages that the page
allocator and kswapd always try to keep free.

The closer (reclaimable pages - dirty pages) is to the number of
reserved pages, the more likely it becomes for reclaim to run into
dirty pages:

+----------+ ---
| anon | |
+----------+ |
| | |
| | -- dirty limit new -- flusher new
| file | | |
| | | |
| | -- dirty limit old -- flusher old
| | |
+----------+ --- reclaim
| reserved |
+----------+
| kernel |
+----------+

Not treating reserved pages as dirtyable on a global level is only a
conceptual fix. In reality, dirty pages are not distributed equally
across zones and reclaim runs into dirty pages on a regular basis.

But it is important to get this right before tackling the problem on a
per-zone level, where the distance between reclaim and the dirty pages
is mostly much smaller in absolute numbers.

Signed-off-by: Johannes Weiner <jweiner@redhat.com>
---
include/linux/mmzone.h | 1 +
mm/page-writeback.c | 8 +++++---
mm/page_alloc.c | 1 +
3 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 1ed4116..e28f8e0 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -316,6 +316,7 @@ struct zone {
* sysctl_lowmem_reserve_ratio sysctl changes.
*/
unsigned long lowmem_reserve[MAX_NR_ZONES];
+ unsigned long totalreserve_pages;

#ifdef CONFIG_NUMA
int node;
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index da6d263..9f896db 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -169,8 +169,9 @@ static unsigned long highmem_dirtyable_memory(unsigned long total)
struct zone *z =
&NODE_DATA(node)->node_zones[ZONE_HIGHMEM];

- x += zone_page_state(z, NR_FREE_PAGES) +
- zone_reclaimable_pages(z);
+ x += zone_page_state(z, NR_FREE_PAGES) -
+ zone->totalreserve_pages;
+ x += zone_reclaimable_pages(z);
}
/*
* Make sure that the number of highmem pages is never larger
@@ -194,7 +195,8 @@ static unsigned long determine_dirtyable_memory(void)
{
unsigned long x;

- x = global_page_state(NR_FREE_PAGES) + global_reclaimable_pages();
+ x = global_page_state(NR_FREE_PAGES) - totalreserve_pages;
+ x += global_reclaimable_pages();

if (!vm_highmem_is_dirtyable)
x -= highmem_dirtyable_memory(x);
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 1dba05e..7e8e2ee 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5075,6 +5075,7 @@ static void calculate_totalreserve_pages(void)

if (max > zone->present_pages)
max = zone->present_pages;
+ zone->totalreserve_pages = max;
reserve_pages += max;
}
}
--
1.7.6