2017-09-27 7:59 GMT+08:00 Andrew Morton [off-list ref]:

On Fri, 22 Sep 2017 07:12:32 +0800 Yafang Shao [off-list ref] wrote:

quoted

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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