Re: [PATCH] mm: add strictlimit knob

[Maxim Patlasov <mpatlasov@parallels.com>]

Hi Andrew,

On 11/05/2013 02:01 AM, Andrew Morton wrote:
> On Fri, 01 Nov 2013 18:31:40 +0400 Maxim Patlasov <MPatlasov@parallels.com> wrote:
>
>> "strictlimit" feature was introduced to enforce per-bdi dirty limits for
>> FUSE which sets bdi max_ratio to 1% by default:
>>
>> http://www.http.com//article.gmane.org/gmane.linux.kernel.mm/105809
>>
>> However the feature can be useful for other relatively slow or untrusted
>> BDIs like USB flash drives and DVD+RW. The patch adds a knob to enable the
>> feature:
>>
>> echo 1 > /sys/class/bdi/X:Y/strictlimit
>>
>> Being enabled, the feature enforces bdi max_ratio limit even if global (10%)
>> dirty limit is not reached. Of course, the effect is not visible until
>> max_ratio is decreased to some reasonable value.
> I suggest replacing "max_ratio" here with the much more informative
> "/sys/class/bdi/X:Y/max_ratio".
>
> Also, Documentation/ABI/testing/sysfs-class-bdi will need an update
> please.

OK, I'll update it, fix patch description and re-send the patch.

>
>>   mm/backing-dev.c |   35 +++++++++++++++++++++++++++++++++++
>>   1 file changed, 35 insertions(+)
>>
> I'm not really sure what to make of the patch.  I assume you tested it
> and observed some effect.  Could you please describe the test setup and
> the effects in some detail?

I plugged 16GB USB-flash in a node with 8GB RAM running 3.12.0-rc7 and 
started writing a huge file by "dd" (from /dev/zero to USB-flash 
mount-point). While writing I was observing "Dirty" counter as reported 
by /proc/meminfo. As expected it stabilized on a level about 1.2GB (15% 
of total RAM). Immediately after dd completed, the "umount" command took 
about 5 minutes. This corresponded to 5MB write throughput of the flash 
drive.

Then I repeated the experiment after setting tunables:

echo 1 > /sys/class/bdi/8\:16/max_ratio
echo 1 > /sys/class/bdi/8\:16/strictlimit

This time, "Dirty" counter became 100 times lesser - about 12MB and 
"umount" took about a second.

Thanks,
Maxim