[PATCH 07/13] writeback: explicit low bound for vm.dirty_ratio

Wu Fengguang
2010-08-05T16:10:58Z

[-- Attachment #1: min-dirty-ratio.patch --]
[-- Type: text/plain, Size: 2224 bytes --]

Force a user visible low bound of 5% for the vm.dirty_ratio interface.

Currently global_dirty_limits() applies a low bound of 5% for
vm_dirty_ratio. This is not very user visible -- if the user sets
vm.dirty_ratio=1, the operation seems to succeed but will be rounded up
to 5% when used.

Another problem is inconsistency: calc_period_shift() uses the plain
vm_dirty_ratio value, which may be a problem when vm.dirty_ratio is set
to < 5 by the user.

CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
kernel/sysctl.c | 3 ++-
mm/page-writeback.c | 10 ++--------
2 files changed, 4 insertions(+), 9 deletions(-)

--- linux-next.orig/kernel/sysctl.c 2010-08-05 22:48:34.000000000 +0800
+++ linux-next/kernel/sysctl.c 2010-08-05 22:48:47.000000000 +0800
@@ -126,6 +126,7 @@ static int ten_thousand = 10000;

/* this is needed for the proc_doulongvec_minmax of vm_dirty_bytes */
static unsigned long dirty_bytes_min = 2 * PAGE_SIZE;
+static int dirty_ratio_min = 5;

/* this is needed for the proc_dointvec_minmax for [fs_]overflow UID and GID */
static int maxolduid = 65535;
@@ -1031,7 +1032,7 @@ static struct ctl_table vm_table[] = {
.maxlen = sizeof(vm_dirty_ratio),
.mode = 0644,
.proc_handler = dirty_ratio_handler,
- .extra1 = &zero,
+ .extra1 = &dirty_ratio_min,
.extra2 = &one_hundred,
},
{
--- linux-next.orig/mm/page-writeback.c 2010-08-05 22:48:42.000000000 +0800
+++ linux-next/mm/page-writeback.c 2010-08-05 22:48:47.000000000 +0800
@@ -415,14 +415,8 @@ void global_dirty_limits(unsigned long *

if (vm_dirty_bytes)
dirty = DIV_ROUND_UP(vm_dirty_bytes, PAGE_SIZE);
- else {
- int dirty_ratio;
-
- dirty_ratio = vm_dirty_ratio;
- if (dirty_ratio < 5)
- dirty_ratio = 5;
- dirty = (dirty_ratio * available_memory) / 100;
- }
+ else
+ dirty = (vm_dirty_ratio * available_memory) / 100;

if (dirty_background_bytes)
background = DIV_ROUND_UP(dirty_background_bytes, PAGE_SIZE);