linux-kernel - Re: [PATCH 1/2] mm: Guard a use of node_reclaim_distance with CONFIFG

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Date:   Fri, 26 Feb 2021 17:31:40 -0800 (PST)
From:   Hugh Dickins <hughd@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
cc:     Palmer Dabbelt <palmer@...belt.com>, atishp@...shpatra.org,
        peterz@...radead.org, srikar@...ux.vnet.ibm.com,
        valentin.schneider@....com, vbabka@...e.cz, mpe@...erman.id.au,
        Palmer Dabbelt <palmerdabbelt@...gle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        kernel-team@...roid.com,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH 1/2] mm: Guard a use of node_reclaim_distance with
 CONFIFG_NUMA

On Fri, 26 Feb 2021, Andrew Morton wrote:
> On Fri, 26 Feb 2021 12:17:20 -0800 Palmer Dabbelt <palmer@...belt.com> wrote:
> > From: Palmer Dabbelt <palmerdabbelt@...gle.com>
> > 
> > This is only useful under CONFIG_NUMA.  IIUC skipping the check is the
> > right thing to do here, as without CONFIG_NUMA there will never be any
> > large node distances on non-NUMA systems.
> > 
> > I expected this to manifest as a link failure under (!CONFIG_NUMA &&
> > CONFIG_TRANSPARENT_HUGE_PAGES), but I'm not actually seeing that.  I
> > think the reference is just getting pruned before it's checked, but I
> > didn't get that from reading the code so I'm worried I'm missing
> > something.
> > 
> > Either way, this is necessary to guard the definition of
> > node_reclaim_distance with CONFIG_NUMA.
> > 
> > Signed-off-by: Palmer Dabbelt <palmerdabbelt@...gle.com>
> > ---
> >  mm/khugepaged.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index a7d6cb912b05..b1bf191c3a54 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -819,8 +819,10 @@ static bool khugepaged_scan_abort(int nid)
> >  	for (i = 0; i < MAX_NUMNODES; i++) {
> >  		if (!khugepaged_node_load[i])
> >  			continue;
> > +#ifdef CONFIG_NUMA
> >  		if (node_distance(nid, i) > node_reclaim_distance)
> >  			return true;
> > +#endif
> >  	}
> >  	return false;
> >  }
> 
> This makes the entire loop a no-op.  Perhaps Kirill can help take a
> look at removing unnecessary code in khugepaged.c when CONFIG_NUMA=n?

First lines of khugepaged_scan_abort() say
	if (!node_reclaim_mode)
		return false;

And include/linux/swap.h says
#ifdef CONFIG_NUMA
extern int node_reclaim_mode;
extern int sysctl_min_unmapped_ratio;
extern int sysctl_min_slab_ratio;
#else
#define node_reclaim_mode 0
#endif

So, no need for an #ifdef CONFIG_NUMA inside khugepaged_scan_abort().

Hugh