Re: Use ZVC counters to establish exact size of dirtyable pages

[Christoph Lameter <clameter@sgi.com>]

On Tue, 13 Feb 2007, Andrew Morton wrote:

> gaaack.
> 
> If CONFIG_HIGHMEM=n and CONFIG_NUMA=n, that definition of `node' is going
> to come after the calculation of `x' and is going to spit a warning.

Ummm... No. If CONFIG_HIGMEM=n then the definition of node is not going
to be included in the source to be compiled.

> Also I think you'll be wanting a += in that first loop.

Correct. What would I do without you?
 
> I believe i386 NUMA is rare as hen's teeth and perhaps we can just forget
> about optimising for it.

I was just trying to avoid complaints..... I'd be glad to drop the stuff. 
Its going to be much simpler that way.
 
> Wanna have another go at this?  Perhaps split it into separate functions or
> something.

Ok. I split out the highmem stuff which makes determine_dirtyable_memory() 
nice and neat.





We can use the global ZVC counters to establish the exact size of the LRU
and the free pages.  This allows a more accurate determination of the dirty
ratio.

This patch will fix the broken ratio calculations if large amounts of
memory are allocated to huge pags or other consumers that do not put the
pages on to the LRU.

Notes:
- I did not add NR_SLAB_RECLAIMABLE to the calculation of the
  dirtyable pages. Those may be reclaimable but they are at this
  point not dirtyable. If NR_SLAB_RECLAIMABLE would be considered
  then a huge number of reclaimable pages would stop writeback
  from occurring.

- This patch used to be in mm as the last one in a series of patches.
  It was removed when Linus updated the treatment of highmem because
  there was a conflict. I updated the patch to follow Linus' approach.
  This patch is neede to fulfill the claims made in the beginning of the
  patchset that is now in Linus' tree.

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>

Index: linux-2.6/mm/page-writeback.c
===================================================================
--- linux-2.6.orig/mm/page-writeback.c	2007-02-12 09:15:22.000000000 -0800
+++ linux-2.6/mm/page-writeback.c	2007-02-13 09:40:04.000000000 -0800
@@ -119,6 +119,38 @@ static void background_writeout(unsigned
  * We make sure that the background writeout level is below the adjusted
  * clamping level.
  */
+
+static unsigned long highmem_dirtyable_memory(void)
+{
+#ifdef CONFIG_HIGHMEM
+	int node;
+	unsigned long x = 0;
+
+	for_each_online_node(node) {
+		struct zone *z =
+			&NODE_DATA(node)->node_zones[ZONE_HIGHMEM];
+
+		x += zone_page_state(z, NR_FREE_PAGES)
+			+ zone_page_state(z, NR_INACTIVE)
+			+ zone_page_state(z, NR_ACTIVE);
+	}
+	return x;
+#else
+	return 0;
+#endif
+}
+
+static unsigned long determine_dirtyable_memory(void)
+{
+	unsigned long x;
+
+	x = global_page_state(NR_FREE_PAGES)
+		+ global_page_state(NR_INACTIVE)
+		+ global_page_state(NR_ACTIVE)
+		- highmem_dirtyable_memory();
+	return x;
+}
+
 static void
 get_dirty_limits(long *pbackground, long *pdirty,
 					struct address_space *mapping)
@@ -128,17 +160,9 @@ get_dirty_limits(long *pbackground, long
 	int unmapped_ratio;
 	long background;
 	long dirty;
-	unsigned long available_memory = vm_total_pages;
+	unsigned long available_memory = determine_dirtyable_memory();
 	struct task_struct *tsk;
 
-#ifdef CONFIG_HIGHMEM
-	/*
-	 * We always exclude high memory from our count.
-	 */
-	available_memory -= totalhigh_pages;
-#endif
-
-
 	unmapped_ratio = 100 - ((global_page_state(NR_FILE_MAPPED) +
 				global_page_state(NR_ANON_PAGES)) * 100) /
 					vm_total_pages;

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