linux-kernel - Re: [v1] mm: khugepaged: avoid overriding min_free

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Message-ID: <db07095f-45d1-8e20-ccc7-0607e57ad91d@linux.microsoft.com>
Date:   Tue, 15 Sep 2020 12:31:56 -0700
From:   Vijay Balakrishna <vijayb@...ux.microsoft.com>
To:     Pavel Tatashin <pasha.tatashin@...een.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Song Liu <songliubraving@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Allen Pais <apais@...rosoft.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>
Subject: Re: [v1] mm: khugepaged: avoid overriding min_free_kbytes set by user

On 9/15/2020 11:27 AM, Pavel Tatashin wrote:
> On Tue, Sep 15, 2020 at 12:55 AM Vijay Balakrishna
> <vijayb@...ux.microsoft.com> wrote:
>>
>> set_recommended_min_free_kbytes need to honor min_free_kbytes set by the
>> user.  Post start-of-day THP enable or memory hotplug operations can
>> lose user specified min_free_kbytes, in particular when it is higher than
>> calculated recommended value.
>>
>> Signed-off-by: Vijay Balakrishna <vijayb@...ux.microsoft.com>
>> Cc: stable@...r.kernel.org
>> ---
>>   mm/khugepaged.c | 8 ++++----
>>   1 file changed, 4 insertions(+), 4 deletions(-)
>>
>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
>> index 4f7107476a6f..b4b753ba411a 100644
>> --- a/mm/khugepaged.c
>> +++ b/mm/khugepaged.c
>> @@ -2253,7 +2253,7 @@ static void set_recommended_min_free_kbytes(void)
>>   {
>>          struct zone *zone;
>>          int nr_zones = 0;
>> -       unsigned long recommended_min;
>> +       int recommended_min;
> 
> Hi Vijay,
> 
> Please explain in log the type change. user_min_free_kbytes is  of
> "int" type and is initialized to -1, so comparing to unsigned leads to
> undesired effect.

Hi Pasha,

I will update log.  I uncovered it when trying to understand why 
one-line change to address issue raised by Kirill didn't work!

> 
> 
>>
>>          for_each_populated_zone(zone) {
>>                  /*
>> @@ -2280,12 +2280,12 @@ static void set_recommended_min_free_kbytes(void)
>>
>>          /* don't ever allow to reserve more than 5% of the lowmem */
>>          recommended_min = min(recommended_min,
>> -                             (unsigned long) nr_free_buffer_pages() / 20);
>> +                             (int) nr_free_buffer_pages() / 20);
>>          recommended_min <<= (PAGE_SHIFT-10);
>>
>> -       if (recommended_min > min_free_kbytes) {
>> +       if (recommended_min > user_min_free_kbytes) {
> 
> Took me a while to understand, but this change is correct. It is the
> same logic that is done in init_per_zone_wmark_min().
> 
> Please join the two patches you have in one patch series.
> Reviewed-by: Pavel Tatashin <pasha.tatashin@...een.com>

Will do.

Thanks,
Vijay
> 
> Thank you,
> Pasha
>