The amount of dirtyable pages should not include the total number of
free pages: there is a number of reserved pages that the page
allocator and kswapd always try to keep free.

The closer (reclaimable pages - dirty pages) is to the number of
reserved pages, the more likely it becomes for reclaim to run into
dirty pages:

       +----------+ ---
       |   anon   |  |
       +----------+  |
       |          |  |
       |          |  -- dirty limit new    -- flusher new
       |   file   |  |                     |
       |          |  |                     |
       |          |  -- dirty limit old    -- flusher old
       |          |                        |
       +----------+                       --- reclaim
       | reserved |
       +----------+
       |  kernel  |
       +----------+

Not treating reserved pages as dirtyable on a global level is only a
conceptual fix.  In reality, dirty pages are not distributed equally
across zones and reclaim runs into dirty pages on a regular basis.

But it is important to get this right before tackling the problem on a
per-zone level, where the distance between reclaim and the dirty pages
is mostly much smaller in absolute numbers.

Signed-off-by: Johannes Weiner <redacted>
---
 include/linux/mmzone.h |    1 +
 mm/page-writeback.c    |    8 +++++---
 mm/page_alloc.c        |    1 +
 3 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 1ed4116..e28f8e0 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h

@@ -316,6 +316,7 @@ struct zone {
 	 * sysctl_lowmem_reserve_ratio sysctl changes.
 	 */
 	unsigned long		lowmem_reserve[MAX_NR_ZONES];
+	unsigned long		totalreserve_pages;
 
 #ifdef CONFIG_NUMA
 	int node;

diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index da6d263..9f896db 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c

@@ -169,8 +169,9 @@ static unsigned long highmem_dirtyable_memory(unsigned long total)
 		struct zone *z =
 			&NODE_DATA(node)->node_zones[ZONE_HIGHMEM];
 
-		x += zone_page_state(z, NR_FREE_PAGES) +
-		     zone_reclaimable_pages(z);
+		x += zone_page_state(z, NR_FREE_PAGES) -
+			zone->totalreserve_pages;
+		x += zone_reclaimable_pages(z);
 	}
 	/*
 	 * Make sure that the number of highmem pages is never larger

@@ -194,7 +195,8 @@ static unsigned long determine_dirtyable_memory(void)
 {
 	unsigned long x;
 
-	x = global_page_state(NR_FREE_PAGES) + global_reclaimable_pages();
+	x = global_page_state(NR_FREE_PAGES) - totalreserve_pages;
+	x += global_reclaimable_pages();
 
 	if (!vm_highmem_is_dirtyable)
 		x -= highmem_dirtyable_memory(x);

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 1dba05e..7e8e2ee 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c

@@ -5075,6 +5075,7 @@ static void calculate_totalreserve_pages(void)
 
 			if (max > zone->present_pages)
 				max = zone->present_pages;
+			zone->totalreserve_pages = max;
 			reserve_pages += max;
 		}
 	}

-- 
1.7.6

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