Re: [PATCH 1/2] mm: Guard a use of node_reclaim_distance with CONFIFG_NUMA

[Palmer Dabbelt]

On Fri, 26 Feb 2021 17:31:40 PST (-0800), hughd@xxxxxxxxxx wrote:
> On Fri, 26 Feb 2021, Andrew Morton wrote:
> > On Fri, 26 Feb 2021 12:17:20 -0800 Palmer Dabbelt <palmer@xxxxxxxxxxx> wrote:
> > > From: Palmer Dabbelt <palmerdabbelt@xxxxxxxxxx>
> > >
> > > 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@xxxxxxxxxx>
> > > ---
> > >  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().

Ah, thanks, I hadn't seen that.  That certainly explains the lack of an 
undefined reference.

That said: do we generally rely on DCE to prune references to undefined 
symbols?  This particular one seems like it'd get reliably deleted, but it 
seems like a fragile thing to do in general.  This kind of stuff would 
certainly make some code easier to write, though.

I don't really care all that much, though, as I was just sending this along due 
to some build failure report from a user that I couldn't reproduce.  It looked 
like they had some out-of-tree stuff, so in this case I'm fine on fixing this 
being their problem.