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>
---
mm/page-writeback.c | 17 +++++++++++++----
1 files changed, 13 insertions(+), 4 deletions(-)
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 830893b..2a6356c 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -194,11 +194,19 @@ static unsigned long highmem_dirtyable_memory(unsigned long total)
unsigned long x = 0;
for_each_node_state(node, N_HIGH_MEMORY) {
+ unsigned long nr_pages;
struct zone *z =
&NODE_DATA(node)->node_zones[ZONE_HIGHMEM];
- x += zone_page_state(z, NR_FREE_PAGES) +
- zone_reclaimable_pages(z) - z->dirty_balance_reserve;
+ nr_pages = zone_page_state(z, NR_FREE_PAGES) +
+ zone_reclaimable_pages(z);
+ /*
+ * Unreclaimable memory (kernel memory or anonymous memory
+ * without swap) can bring down the dirtyable pages below
+ * the zone's dirty balance reserve.
+ */
+ if (nr_pages >= z->dirty_balance_reserve)
+ x += nr_pages - z->dirty_balance_reserve;
}
/*
* Make sure that the number of highmem pages is never larger
@@ -222,8 +230,9 @@ 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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