Re: [PATCH v4] mm: introduce validity check on vm dirtiness settings

[Yafang Shao <laoar.shao@gmail.com>]

2017-09-27 7:59 GMT+08:00 Andrew Morton <akpm@linux-foundation.org>:
> On Fri, 22 Sep 2017 07:12:32 +0800 Yafang Shao <laoar.shao@gmail.com> wrote:
>
>> we can find the logic in domain_dirty_limits() that
>> when dirty bg_thresh is bigger than dirty thresh,
>> bg_thresh will be set as thresh * 1 / 2.
>>       if (bg_thresh >= thresh)
>>               bg_thresh = thresh / 2;
>>
>> But actually we can set vm background dirtiness bigger than
>> vm dirtiness successfully. This behavior may mislead us.
>> We'd better do this validity check at the beginning.
>>
>> ...
>>
>> --- a/Documentation/sysctl/vm.txt
>> +++ b/Documentation/sysctl/vm.txt
>> @@ -156,6 +156,9 @@ read.
>>  Note: the minimum value allowed for dirty_bytes is two pages (in bytes); any
>>  value lower than this limit will be ignored and the old configuration will be
>>  retained.
>> +Note: the value of dirty_bytes also cannot be set lower than
>> +dirty_background_bytes or the amount of memory corresponding to
>> +dirty_background_ratio.
>
> I think this means that a script which alters both dirty_bytes and
> dirty_background_bytes must alter dirty_background_bytes first if they
> are being decreased and must alter dirty_bytes first if they are being
> increased.  Or something like that.
>

Yes.

> And existing scripts which do not do this will cease to work correctly,
> no?
>

The existing scritpts won't work correctly. That's also what I have
worried before.

But under this condition, there's a error message generated by "sysctl
-w" to tell them the first setting was failure.
This error message may be a reminder to them that there are some
connections between background and direct limit, and should not set
arbitrary.
May that's better. I'm not sure.

Thanks
Yafang

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