[PATCH 12/27] nfs: lower writeback threshold proportionally to dirty threshold

[Wu Fengguang <fengguang.wu@intel.com>]

[-- Attachment #1: nfs-congestion-thresh.patch --]
[-- Type: text/plain, Size: 1985 bytes --]

nfs_congestion_kb is to control the max allowed writeback and in-commit
pages. It's not reasonable for them to outnumber dirty and to-commit
pages. So each of them should not take more than 1/4 dirty threshold.

Considering that nfs_init_writepagecache() is called on fresh boot,
at the time dirty_thresh is much higher than the real dirty limit after
lots of user space memory consumptions, use 1/8 instead.

We might update nfs_congestion_kb when global dirty limit is changed
at runtime, but whatever, do it simple first.

CC: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 fs/nfs/write.c      |   13 +++++++++++++
 mm/page-writeback.c |    1 +
 2 files changed, 14 insertions(+)

--- linux-next.orig/fs/nfs/write.c	2011-03-03 14:04:01.000000000 +0800
+++ linux-next/fs/nfs/write.c	2011-03-03 14:04:01.000000000 +0800
@@ -1651,6 +1651,9 @@ out:
 
 int __init nfs_init_writepagecache(void)
 {
+	unsigned long background_thresh;
+	unsigned long dirty_thresh;
+
 	nfs_wdata_cachep = kmem_cache_create("nfs_write_data",
 					     sizeof(struct nfs_write_data),
 					     0, SLAB_HWCACHE_ALIGN,
@@ -1688,6 +1691,16 @@ int __init nfs_init_writepagecache(void)
 	if (nfs_congestion_kb > 256*1024)
 		nfs_congestion_kb = 256*1024;
 
+	/*
+	 * Limit to 1/8 dirty threshold, so that writeback+in_commit pages
+	 * won't overnumber dirty+to_commit pages.
+	 */
+	global_dirty_limits(&background_thresh, &dirty_thresh);
+	dirty_thresh <<= PAGE_SHIFT - 10;
+
+	if (nfs_congestion_kb > dirty_thresh / 8)
+		nfs_congestion_kb = dirty_thresh / 8;
+
 	return 0;
 }
 
--- linux-next.orig/mm/page-writeback.c	2011-03-03 14:04:01.000000000 +0800
+++ linux-next/mm/page-writeback.c	2011-03-03 14:04:01.000000000 +0800
@@ -431,6 +431,7 @@ void global_dirty_limits(unsigned long *
 	*pbackground = background;
 	*pdirty = dirty;
 }
+EXPORT_SYMBOL_GPL(global_dirty_limits);
 
 /**
  * bdi_dirty_limit - @bdi's share of dirty throttling threshold