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

[Michal Hocko]

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>

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.
-- 
Michal Hocko
SUSE Labs