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 19:05:43 -0800 (PST)
From:   Palmer Dabbelt <palmer@...belt.com>
To:     hughd@...gle.com
CC:     akpm@...ux-foundation.org, atishp@...shpatra.org,
        peterz@...radead.org, srikar@...ux.vnet.ibm.com,
        valentin.schneider@....com, vbabka@...e.cz, mpe@...erman.id.au,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        kernel-team@...roid.com, 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 17:31:40 PST (-0800), hughd@...gle.com wrote:
> 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().

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.