Re: [v1] mm: khugepaged: avoid overriding min_free_kbytes set by user

[Vijay Balakrishna]

On 9/15/2020 11:27 AM, Pavel Tatashin wrote:
> On Tue, Sep 15, 2020 at 12:55 AM Vijay Balakrishna
> <vijayb@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > ---
> >   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@xxxxxxxxxx>

Will do.

Thanks,
Vijay
> Thank you,
> Pasha