Re: [v3 1/2] mm: khugepaged: recalculate min_free_kbytes after memory hotplug as expected by khugepaged

[Vijay Balakrishna]

On 9/25/2020 12:42 AM, Michal Hocko wrote:
> On Wed 16-09-20 18:21:48, Vijay Balakrishna wrote:
> > When memory is hotplug added or removed the min_free_kbytes must be
> > recalculated based on what is expected by khugepaged.  Currently
> > after hotplug, min_free_kbytes will be set to a lower default and higher
> > default set when THP enabled is lost.  This change restores min_free_kbytes
> > as expected for THP consumers.
> >
> > Fixes: f000565adb77 ("thp: set recommended min free kbytes")
> >
> > Signed-off-by: Vijay Balakrishna <vijayb@xxxxxxxxxxxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > Reviewed-by: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
>
> I am ok with this patch. I am not sure this is worth backporting to
> stable trees becasuse this is not a functional bug. Surprising behavior,
> yes, but not much more than that.
>
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>

Thanks Michal.

> One minor comment below
> [...]
> > @@ -857,6 +858,7 @@ int __ref online_pages(unsigned long pfn, unsigned long nr_pages,
> >   	zone_pcp_update(zone);
> >   
> >   	init_per_zone_wmark_min();
> > +	khugepaged_min_free_kbytes_update();
> >   
> >   	kswapd_run(nid);
> >   	kcompactd_run(nid);
> > @@ -1600,6 +1602,7 @@ static int __ref __offline_pages(unsigned long start_pfn,
> >   	pgdat_resize_unlock(zone->zone_pgdat, &flags);
> >   
> >   	init_per_zone_wmark_min();
> > +	khugepaged_min_free_kbytes_update();
> >   
> >   	if (!populated_zone(zone)) {
> >   		zone_pcp_reset(zone);
>
> Can we move khugepaged_min_free_kbytes_update into
> init_per_zone_wmark_min? If it stays external we might hit the same
> problem when somebody else needs to modify min_free_kbytes. Early init
> call will be likely too early for khugepaged but that shouldn't matter
> AFAICS because it will call khugepaged_min_free_kbytes_update on its
> own.

Sure, let me take a look and post v4 next week.

Thanks,
Vijay

>