Re: [RFC] mm: show message when updating min_free_kbytes in thp

Michal Hocko
2014-01-10T08:05:11Z

On Thu 09-01-14 13:15:54, David Rientjes wrote:
> On Thu, 9 Jan 2014, Han Pingtian wrote:
>
> > min_free_kbytes may be raised during THP's initialization. Sometimes,
> > this will change the value being set by user. Showing message will
> > clarify this confusion.
> >
> > Only show this message when changing the value set by user according to
> > Michal Hocko's suggestion.
> >
> > Showing the old value of min_free_kbytes according to Dave Hansen's
> > suggestion. This will give user the chance to restore old value of
> > min_free_kbytes.
> >
> > Signed-off-by: Han Pingtian <hanpt@linux.vnet.ibm.com>
> > ---
> > mm/huge_memory.c | 9 ++++++++-
> > mm/page_alloc.c | 2 +-
> > 2 files changed, 9 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 7de1bf8..e0e4e29 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -100,6 +100,7 @@ static struct khugepaged_scan khugepaged_scan = {
> > .mm_head = LIST_HEAD_INIT(khugepaged_scan.mm_head),
> > };
> >
> > +extern int user_min_free_kbytes;
> >
>
> We don't add extern declarations to .c files. How many other examples of
> this can you find in mm/?

I have suggested this because general visibility is not needed. But if
you think that it should then include/linux/mm.h sounds like a proper
place.

> > static int set_recommended_min_free_kbytes(void)
> > {
> > @@ -130,8 +131,14 @@ static int set_recommended_min_free_kbytes(void)
> > (unsigned long) nr_free_buffer_pages() / 20);
> > recommended_min <<= (PAGE_SHIFT-10);
> >
> > - if (recommended_min > min_free_kbytes)
> > + if (recommended_min > min_free_kbytes) {
> > + if (user_min_free_kbytes >= 0)
> > + pr_info("raising min_free_kbytes from %d to %lu "
> > + "to help transparent hugepage allocations\n",
> > + min_free_kbytes, recommended_min);
> > +
> > min_free_kbytes = recommended_min;
> > + }
> > setup_per_zone_wmarks();
> > return 0;
> > }
>
> Does this even ever trigger since set_recommended_min_free_kbytes() is
> called via late_initcall()?

This is called whenever THP is enabled so it might be called later after
boot. The point is AFAIU to warn user that the admin decision about
min_free_kbytes is overridden.

--
Michal Hocko
SUSE Labs