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Subject: Re: [v3 1/2] mm: khugepaged: recalculate min_free_kbytes after memory
 hotplug as expected by khugepaged
To:     Michal Hocko <mhocko@suse.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Song Liu <songliubraving@fb.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Pavel Tatashin <pasha.tatashin@soleen.com>,
        Allen Pais <apais@microsoft.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
References: <1600305709-2319-1-git-send-email-vijayb@linux.microsoft.com>
 <1600305709-2319-2-git-send-email-vijayb@linux.microsoft.com>
 <20200925074215.GA3389@dhcp22.suse.cz>
From:   Vijay Balakrishna <vijayb@linux.microsoft.com>
Message-ID: <751cd286-5919-fc0a-21b4-75460934e305@linux.microsoft.com>
Date:   Fri, 25 Sep 2020 09:31:39 -0700
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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@linux.microsoft.com>
>> Cc: stable@vger.kernel.org
>> Reviewed-by: Pavel Tatashin <pasha.tatashin@soleen.com>
> 
> 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@suse.com>

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

>