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Date:   Fri, 26 Feb 2021 20:14:47 -0800 (PST)
Subject:     Re: [PATCH 1/2] mm: Guard a use of node_reclaim_distance with CONFIFG_NUMA
In-Reply-To: <alpine.LSU.2.11.2102261918250.15774@eggly.anvils>
CC:     hughd@google.com, akpm@linux-foundation.org, atishp@atishpatra.org,
        peterz@infradead.org, srikar@linux.vnet.ibm.com,
        valentin.schneider@arm.com, vbabka@suse.cz, mpe@ellerman.id.au,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        kernel-team@android.com, kirill.shutemov@linux.intel.com
From:   Palmer Dabbelt <palmer@dabbelt.com>
To:     hughd@google.com
Message-ID: <mhng-3f22abd3-c7a3-4a5f-b068-ed5bfe9a2e71@penguin>
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On Fri, 26 Feb 2021 19:41:40 PST (-0800), hughd@google.com wrote:
> On Fri, 26 Feb 2021, Palmer Dabbelt wrote:
>> On Fri, 26 Feb 2021 17:31:40 PST (-0800), hughd@google.com wrote:
>> > On Fri, 26 Feb 2021, Andrew Morton wrote:
>> > > On Fri, 26 Feb 2021 12:17:20 -0800 Palmer Dabbelt <palmer@dabbelt.com>
>> > > wrote:
>> > > > From: Palmer Dabbelt <palmerdabbelt@google.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@google.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.
>
> Yes, the kernel build very much depends on the optimizer eliminating
> dead code, in many many places.  We do prefer to keep the #ifdefs to
> the header files as much as possible.

OK, makes sense.  Thanks!

>> 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.
>
> I didn't see your 2/2 at the time; but wouldn't be surprised if that
> needs 1/2, to avoid an error on undeclared node_reclaim_distance before
> the optimizer comes into play.  If so, best just to drop 2/2 too.

Ya, definitely.  Sorry for the noise!