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

Michal Hocko
2020-09-16T07:59:08Z

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.

> 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.

> 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
>

--
Michal Hocko
SUSE Labs