Re: [BUG] DAMON sysfs: failed update clears tried_regions snapshot

[Aveline Noir]

Hi SJ,

I haven't seen this affect a real workload. I found it while testing
the DAMON sysfs interface.

If a tool tries to refresh after monitoring stops, the failed request
can delete the last snapshot. The tool then has no previous result to
fall back to.

I built and tested 66178e4ec30a and its parent cce0b334c310 with the
same config and reproducer. In three runs each, the parent kept the
4 MiB snapshot after rejecting the refresh; 66178e4ec30a cleared it.
Both returned -EINVAL.

Thanks,
Aveline Noir


On Thu, 15 Oct 2026 07:05:01 -0700, SJ Park <sj@xxxxxxxxxx> wrote:
> Hi Avelin,
>
> On Thu, 15 Oct 2026 07:22:38 -0400 Aveline Noir <jm5905938@xxxxxxxxx> wrote:
>
> > Hi,
> >
> > On mm-new commit ad9f2d463666, requesting
> > update_schemes_tried_regions after stopping a kdamond returns
> > -ECANCELED and clears the previous tried_regions snapshot.  The final
> > result collected before "off" is then lost.
>
> Could you further share your use case that this behavior becomes a real
> problem?  Also, have you tried bisecting this to find from when this behavior
> was introduced?  If so, could you share the commit that introduced this
> behavior?
>
>
> THanks,
> SJ
>
> [...]