linux-kernel - Re: [RFC] mm: show message when updating min_free

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Message-ID: <20140110080504.GA9437@dhcp22.suse.cz>
Date:	Fri, 10 Jan 2014 09:05:04 +0100
From:	Michal Hocko <mhocko@...e.cz>
To:	David Rientjes <rientjes@...gle.com>
Cc:	linux-kernel@...r.kernel.org, Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	Dave Hansen <dave.hansen@...el.com>
Subject: Re: [RFC] mm: show message when updating min_free_kbytes in thp

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@...ux.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
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