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

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Date:   Tue, 15 Sep 2020 14:27:34 -0400
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     Vijay Balakrishna <vijayb@...ux.microsoft.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 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.


>
>         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>

Thank you,
Pasha