Re: [RFC PATCH 0/4] mm/damon: introduce perf event based access check

[SeongJae Park <sj@kernel.org>]

On Tue, 27 Jan 2026 17:12:23 -0800 SeongJae Park <sj@kernel.org> wrote:

> On Tue, 27 Jan 2026 21:56:43 +0900 Akinobu Mita <akinobu.mita@gmail.com> wrote:
> 
> > 2026年1月27日(火) 15:43 SeongJae Park <sj@kernel.org>:
> > > Actually DAMON is internally setting such maximum region size based on the
> > > min_nr_regions parameter, via damon_region_sz_limit().  Nonetheless, the limit
> > > is applied only in regions merge time.  That's why it requires the regions
> > > split to happen sufficiently until the real fixed granularity monitoring is
> > > started.
> > >
> > > And I think this behavior is just a bug, or suboptimum implementation at least.
> > > That is, users set the minimum number of regions but it may not really be kept.
> > > That's definitely confusing behavior.  Actually there was a similar case that
> > > number of regions can be larger than the max_nr_regions.  We fixed it, with
> > > commit 310d6c15e910 ("mm/damon/core: merge regions aggressively when
> > > max_nr_regions is unmet").  I think we discussed about similar case for
> > > min_nr_regions, but I cannot find the discussion for now.
> > >
> > > So, I think it is better to fix this rather than introducing a new parameter.
> > 
> > I agree with that.
> > 
> > > Maybe we can split regions based on the min_nr_regions based size limit, before
> > > starting the main loop of kdamond_fn().  Similar to the max_nr_regions
> > > violation, there could be yet another corner case on online parameters commit
> > > situation, so it would better to check the case, too.  You could implement such
> > > fix on your own, or let me do that.  In the latter case, if you don't mind, I
> > > will add your Reported-by: tag to the fix.  Please let me know your
> > > preferrence.
> > 
> > You'll be better able to fix it, so please fix it at your convenience.
> > Adding Reported-by: tag is fine.
> 
> Sounds good, I will do so!

I just posted an RFC patch series [1] for this.  I will drop RFC tag after the
current merge window is finished.  Please let me know if you find something
wrong there!

[1] https://lore.kernel.org/20260217000400.69056-1-sj@kernel.org


Thanks,
SJ

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