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 <sonny...@chromium.org>
Signed-off-by: Puneet Kumar <puneets...@chromium.org>
Acked-by: Johannes Weiner <han...@cmpxchg.org>
CC: sta...@vger.kernel.org
---
v2: added apkm's suggestion to make the highmem calculation better
v3: added Fengguang Wu's suggestions fix zone_dirtyable_memory() and
(offlist mail) to use max() in global_dirtyable_memory()
v4: Added suggestions to description clarifying the role of highmem
and the commit which originally caused the problem
mm/page-writeback.c | 25 ++++++++++++++++++++-----
1 files changed, 20 insertions(+), 5 deletions(-)
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 830893b..f9efbe8 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -201,6 +201,18 @@ static unsigned long highmem_dirtyable_memory(unsigned
long total)
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_memory(void)
{
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 -= max(x, dirty_balance_reserve);
if (!vm_highmem_is_dirtyable)
x -= highmem_dirtyable_memory(x);
@@ -290,9 +302,12 @@ static unsigned long zone_dirtyable_memory(struct zone
*zone)
* 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 -= max(nr_pages, zone->dirty_balance_reserve);
+ return nr_pages;
}
/**
--
1.7.7.3
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