On Mon, Aug 23, 2010 at 12:42:48PM +0800, Neil Brown wrote:
> On Fri, 20 Aug 2010 15:50:54 +1000
> Con Kolivas <kernel@xxxxxxxxxxx> wrote:
>
> > On Fri, 20 Aug 2010 02:13:25 pm KOSAKI Motohiro wrote:
> > > > The dirty_ratio was silently limited to >= 5%. This is not a user
> > > > expected behavior. Let's rip it.
> > > >
> > > > It's not likely the user space will depend on the old behavior.
> > > > So the risk of breaking user space is very low.
> > > >
> > > > CC: Jan Kara <jack@xxxxxxx>
> > > > CC: Neil Brown <neilb@xxxxxxx>
> > > > Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> > >
> > > Thank you.
> > > Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> >
> > I have tried to do this in the past, and setting this value to 0 on some
> > machines caused the machine to come to a complete standstill with small
> > writes to disk. It seemed there was some kind of "minimum" amount of data
> > required by the VM before anything would make it to the disk and I never
> > quite found out where that blockade occurred. This was some time ago (3 years
> > ago) so I'm not sure if the problem has since been fixed in the VM since
> > then. I suggest you do some testing with this value set to zero before
> > approving this change.
You are right, vm.dirty_ratio=0 will block applications for ever..
>
> If it is appropriate to have a lower limit, that should be imposed where
> the sysctl is defined in kernel/sysctl.c, not imposed after the fact where
> the value is used.
>
> As we now have dirty_bytes which over-rides dirty_ratio, there is little
> cost in having a lower_limit for dirty_ratio - it could even stay at 5% -
> but it really shouldn't be silent. Writing a number below the limit to the
> sysctl file should fail.
How about imposing an explicit bound of 1%? That's more natural and
its risk of breaking user space should be lower than 5%.
Thanks,
Fengguang
---
writeback: remove the internal 5% low bound on dirty_ratio
The dirty_ratio was silently limited in global_dirty_limits() to >= 5%. This
is not a user expected behavior. And it's inconsistent with calc_period_shift(),
which uses the plain vm_dirty_ratio value. So let's rip the internal bound.
At the same time, force a user visible low bound of 1% for the vm.dirty_ratio
interface. Applications trying to write 0 will be rejected with -EINVAL. This
will break user space applications if they
1) try to write 0 to vm.dirty_ratio
2) and check the return value
That is very weird combination, so the risk of breaking user space is low.
CC: Jan Kara <jack@xxxxxxx>
CC: Neil Brown <neilb@xxxxxxx>
CC: Rik van Riel <riel@xxxxxxxxxx>
CC: Con Kolivas <kernel@xxxxxxxxxxx>
CC: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
CC: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
kernel/sysctl.c | 2 +-
mm/page-writeback.c | 10 ++--------
2 files changed, 3 insertions(+), 9 deletions(-)
--- linux-next.orig/mm/page-writeback.c 2010-08-20 20:14:11.000000000 +0800
+++ linux-next/mm/page-writeback.c 2010-08-23 10:31:01.000000000 +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);
--- linux-next.orig/kernel/sysctl.c 2010-08-23 14:06:11.000000000 +0800
+++ linux-next/kernel/sysctl.c 2010-08-23 14:07:30.000000000 +0800
@@ -1029,7 +1029,7 @@ static struct ctl_table vm_table[] = {
.maxlen = sizeof(vm_dirty_ratio),
.mode = 0644,
.proc_handler = dirty_ratio_handler,
- .extra1 = &zero,
+ .extra1 = &one,
.extra2 = &one_hundred,
},
{
--
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