The patch titled
Subject: mm: fix calculation of dirtyable memory
has been added to the -mm tree. Its filename is
mm-fix-calculation-of-dirtyable-memory.patch
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From: Sonny Rao <sonnyrao@xxxxxxxxxxxx>
Subject: mm: fix calculation of dirtyable memory
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@xxxxxxxxxxxx>
Signed-off-by: Puneet Kumar <puneetster@xxxxxxxxxxxx>
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Tested-by: Damien Wyart <damien.wyart@xxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/page-writeback.c | 25 ++++++++++++++++++++-----
1 file changed, 20 insertions(+), 5 deletions(-)
diff -puN mm/page-writeback.c~mm-fix-calculation-of-dirtyable-memory mm/page-writeback.c
--- a/mm/page-writeback.c~mm-fix-calculation-of-dirtyable-memory
+++ a/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;
}
/**
_
Patches currently in -mm which might be from sonnyrao@xxxxxxxxxxxx are
mm-fix-calculation-of-dirtyable-memory.patch
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