Messages in this thread | | | 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 | From | Palmer Dabbelt <> |
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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!
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