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

Vijay Balakrishna
2020-09-16T19:43:10Z

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
>>
>