[ 015/123] mm: fix calculation of dirtyable memory

Greg Kroah-Hartman
2013-01-09T20:37:16Z

From: Sonny Rao <sonnyrao@chromium.org>

commit c8b74c2f6604923de91f8aa6539f8bb934736754 upstream.

The system uses global_dirtyable_memory() to calculate number of
dirtyable pages/pages that can be allocated to the page cache. A bug
causes an underflow thus making the page count look like a big unsigned
number. This in turn confuses the dirty writeback throttling to
aggressively write back pages as they become dirty (usually 1 page at a
time). This generally only affects systems with highmem because the
underflowed count gets subtracted from the global count of dirtyable
memory.

The problem was introduced with v3.2-4896-gab8fabd

Fix is to ensure we don't get an underflowed total of either highmem or
global dirtyable memory.

Signed-off-by: Sonny Rao <sonnyrao@chromium.org>
Signed-off-by: Puneet Kumar <puneetster@chromium.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Tested-by: Damien Wyart <damien.wyart@free.fr>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
mm/page-writeback.c | 25 ++++++++++++++++++++-----
1 file changed, 20 insertions(+), 5 deletions(-)

--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -201,6 +201,18 @@ static unsigned long highmem_dirtyable_m
zone_reclaimable_pages(z) - z->dirty_balance_reserve;
}
/*
+ * Unreclaimable memory (kernel memory or anonymous memory
+ * without swap) can bring down the dirtyable pages below
+ * the zone's dirty balance reserve and the above calculation
+ * will underflow. However we still want to add in nodes
+ * which are below threshold (negative values) to get a more
+ * accurate calculation but make sure that the total never
+ * underflows.
+ */
+ if ((long)x < 0)
+ x = 0;
+
+ /*
* Make sure that the number of highmem pages is never larger
* than the number of the total dirtyable memory. This can only
* occur in very strange VM situations but we want to make sure
@@ -222,8 +234,8 @@ static unsigned long global_dirtyable_me
{
unsigned long x;

- x = global_page_state(NR_FREE_PAGES) + global_reclaimable_pages() -
- dirty_balance_reserve;
+ x = global_page_state(NR_FREE_PAGES) + global_reclaimable_pages();
+ x -= min(x, dirty_balance_reserve);

if (!vm_highmem_is_dirtyable)
x -= highmem_dirtyable_memory(x);
@@ -290,9 +302,12 @@ static unsigned long zone_dirtyable_memo
* highmem zone can hold its share of dirty pages, so we don't
* care about vm_highmem_is_dirtyable here.
*/
- return zone_page_state(zone, NR_FREE_PAGES) +
- zone_reclaimable_pages(zone) -
- zone->dirty_balance_reserve;
+ unsigned long nr_pages = zone_page_state(zone, NR_FREE_PAGES) +
+ zone_reclaimable_pages(zone);
+
+ /* don't allow this to underflow */
+ nr_pages -= min(nr_pages, zone->dirty_balance_reserve);
+ return nr_pages;
}

/**