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

[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.

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

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