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).
Fix is to ensure there is no underflow while doing the math.
Signed-off-by: Sonny Rao <sonnyrao@xxxxxxxxxxxx>
Signed-off-by: Puneet Kumar <puneetster@xxxxxxxxxxxx>
---
v2: added apkm's suggestion to make the highmem calculation better
mm/page-writeback.c | 17 +++++++++++++++--
1 files changed, 15 insertions(+), 2 deletions(-)
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 830893b..ce62442 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 +234,9 @@ 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();
+ if (x >= dirty_balance_reserve)
+ x -= dirty_balance_reserve;
if (!vm_highmem_is_dirtyable)
x -= highmem_dirtyable_memory(x);
--
1.7.7.3
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