Re: [PATCH] mm/damon/core: reject empty access pattern intervals on scheme commit

[SJ Park <sj@kernel.org>]

On Tue,  3 Nov 2026 18:54:18 +0800 "Xiaolei Wang (Kaihong)" <xiaoleiwangxiaolei@gmail.com> wrote:

> DAMOS schemes accept access pattern bounds with min > max (e.g.
> min_sz_region > max_sz_region).  No input path validates the bounds:
> damon_new_scheme() stores the pattern as is, and the sysfs ul_range
> stores are plain kstrtoul().
> 
> At runtime __damos_valid_target() evaluates "min <= x <= max" for each
> of the three pattern dimensions.  A reversed interval is an empty set,
> so every region fails the check and the scheme silently never matches
> anything: nr_tried stays zero forever while the kdamond keeps running
> normally.
> 
> With such a reversed interval, damo starts normally (rc=0, kdamond
> state=on), but nr_tried and nr_applied stay zero forever and the target
> RSS does not drop.
> 
> The same holds for the watermarks thresholds, which must be monotonic
> (low <= mid <= high).
> 
> Validate the access pattern and watermarks at the beginning of
> damos_commit(), through which both new schemes (damo start) and online
> updates (damo tune) pass.

The documentation is not explicitly mentioning about the inputs (min > max and
non-monotonic watermarks).  But I was thinking it is not really difficult to
infer this behavior based on the existing document.

If my understanding of the documentation is not wrong, I find no problem at
accepting the inputs and just behave as documented.  It might look silly, but
who knows the users have a creative reason to do that?  As long as it is not
causing a harm, just supporting it is better for both users and code
maintainability, in my humble opinion.

Pleae let me know if I am missing something.


Thanks,
SJ

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