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@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>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 20 insertions, 5 deletions

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 6f4271224493..0713bfbf0954 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 -= min(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 -= min(nr_pages, zone->dirty_balance_reserve);
+        return nr_pages;
 }
 
 /**