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

[Vijay Balakrishna]

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@xxxxxxxxxxxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > Reviewed-by: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
> > ---
> >   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