Re: [v2 2/2] mm: khugepaged: avoid overriding min_free_kbytes set by user

[Vijay Balakrishna <vijayb@linux.microsoft.com>]

On 9/16/2020 12:59 AM, Michal Hocko wrote:
> On Tue 15-09-20 14:10:58, Vijay Balakrishna 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.  Also modifying "recommended_min" variable
>> type to "int" from "unsigned long" to avoid undesired result noticed
>> during testing.  It is due to comparing "unsigned long" with "int" type.
>>
>> Signed-off-by: Vijay Balakrishna <vijayb@linux.microsoft.com>
>> Cc: stable@vger.kernel.org
>> Reviewed-by: Pavel Tatashin <pasha.tatashin@soleen.com>
>> ---
>>   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;
>>   
>>   	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);
> 
> nr_free_buffer_pages can oveflow in int on very large machines.

Good point.  I will address it.

> 
>>   	recommended_min <<= (PAGE_SHIFT-10);
>>   
>> -	if (recommended_min > min_free_kbytes) {
>> +	if (recommended_min > user_min_free_kbytes) {
> 
> This can decrease the size theoretically. Because user_min_free_kbytes
> is -1 by default and recommended_min might be <= min_free_kbytes.
> 
> You need to check both. Also can we make user_min_free_kbytes 0 by
> default? From a quick look, nobody should really care.

Let me rework.

Thanks,
Vijay

> 
>>   		if (user_min_free_kbytes >= 0)
>> -			pr_info("raising min_free_kbytes from %d to %lu to help transparent hugepage allocations\n",
>> +			pr_info("raising min_free_kbytes from %d to %d to help transparent hugepage allocations\n",
>>   				min_free_kbytes, recommended_min);
>>   
>>   		min_free_kbytes = recommended_min;
>> -- 
>> 2.28.0
>>
>