Re: [PATCH] mm/damon: update expired description of damos_action

[SeongJae Park <sj@kernel.org>]

On Sun, 3 Aug 2025 11:03:12 +0900 Honggyu Kim <honggyu.kim@sk.com> wrote:

> Hi SeongJae and Sang-Heon,
> 
> On 8/2/2025 1:50 AM, SeongJae Park wrote:
> > On Sat, 2 Aug 2025 01:11:09 +0900 Sang-Heon Jeon <ekffu200098@gmail.com> wrote:
> > 
> >> Hi, Honggyu
> >>
> >> On Fri, Aug 1, 2025 at 8:35 PM Honggyu Kim <honggyu.kim@sk.com> wrote:
> >>>
> >>> Hi Sang-Heon and SeongJae,
> >>>
> >>> On 8/1/2025 2:58 AM, SeongJae Park wrote:
> >>>> Hello Sang-Heon,
> >>>>
> >>>> On Thu, 31 Jul 2025 22:22:30 +0900 Sang-Heon Jeon <ekffu200098@gmail.com> wrote:
> >>>>
> >>>>> Nowadays, damos operation actions support more various operation set.
> >>>>> But comments(also, generated documentation) doesn't updated.
> >>>>> So, fix the comments with current support status.
> > [...]
> >>>>> diff --git a/include/linux/damon.h b/include/linux/damon.h
> > [...]
> >>>>>     * @DAMOS_WILLNEED:  Call ``madvise()`` for the region with MADV_WILLNEED.
> >>>>>     * @DAMOS_COLD:              Call ``madvise()`` for the region with MADV_COLD.
> >>>>> - * @DAMOS_PAGEOUT:   Call ``madvise()`` for the region with MADV_PAGEOUT.
> >>>>> + * @DAMOS_PAGEOUT:   Reclaim the region.
> >>>>
> >>>> Nice!
> >>>
> >>> But doesn't it make confusion about whether this pages out to disk or does
> >>> demotion to the lower tier memory?  It's because PAGEOUT action doesn't do
> >>> demotion, but it looks "reclaim" includes pageout and demotion together in my
> >>> understanding since /sys/kernel/mm/numa/demotion_enabled was introduced.
> > 
> > To my understanding, DAMOS_PAGEOUT can also do demotion when demotion_enabled
> > is set.  Am I missing something?
> 
> Actually no, please see below.

I'm unsure to what point you are saying "no".  Are you saying DAMOS_PAGEOUT can
also do demotion when demotion_enabled is set?  Or not?  Could you please
clarify, and add more explanations about why you think so?

> 
> do_demote_pass in shrink_folio_list()
> https://github.com/torvalds/linux/blob/v6.16/mm/vmscan.c#L1122
> 
> The do_demote_pass is used here.
> https://github.com/torvalds/linux/blob/v6.16/mm/vmscan.c#L1293-L1302
> 
> can_demote() implementation returns false when demotion_enabled is on.
> https://github.com/torvalds/linux/blob/v6.16/mm/vmscan.c#L350-L351

I'm again get confused.  Isn't it opposite?

```
static bool can_demote(int nid, struct scan_control *sc,
		       struct mem_cgroup *memcg)
{
	int demotion_nid;

	if (!numa_demotion_enabled)
		return false;
```

It returns "false" when demotion_enabled is "off" (unset).  Am I reading
something wrong...?

> 
> The replated commit is as follows.
> mm/migrate: add sysfs interface to enable reclaim migration
> https://github.com/torvalds/linux/commit/20b51af15e014cac63b58a4f8b8b323ac35bccce
> 
> > 
> >>
> >> My intention was just to synchronize with the Design documentation.
> >>
> >> So how about changing the description to `Page out the region`, Would
> >> this be also confusing?
> >> I feel like it would be clearer than using word "reclaim"
> 
> I don't have a good idea but it looks like recusive explanation.
> 
> > 
> > In my opinion, "reclaim" is good.
> 
> I wish there could be better term that distinguishes between swap out and
> demotion.  In my understanding "reclaim" includes both swap out and demotion.

"Reclaim" also includes writeback operations.

I agree we have many rooms to improve in terms of terminologies.  But, I'd
argue we don't need to have only 1:1 mapping terminologies.  Othrwise, maybe we
don't need any documentation at all but just code.  "Reclaim" is a good general
terminology for describing an effort to get free pages on a memory domain (NUMA
node, zone, etc), in my opinion.

To be honest, btw, I'm not a fan of "promote/demote", and that was one of the
reasons I insisted "DAMOS_MIGRATE_{HOT,COLD}" instead of
"DAMOS_{PROMOTE,DEMOTE}".

> 
> I also found that man page explanation about MADV_PAGEOUT is "reclaim these 
> pages".  If this is correct, then maybe demotion isn't included in "reclaim".

I interpret the term "reclaim" on the man page with a flexibility including my
above definition, and hence I don't think this is contradicting in a very bad
way against what I'm understanding.  That is, I interpret the documentation
says MADV_PAGEOUT can also do demote pages under certain conditions.

I didn't write the documentation, so I may be completely wrong.  I also
frustratingly lost my resource to validate the main question of this discussion
(whether DAMOS_PAGEOUT can also do demotion or not), for now.  I'd like to
continue this discussion based on code rather than a documentation that _might_
be right or wrong, and preferrably based on real tests if you or I have a good
testing setup.


Thanks,
SJ

[...]