Re: [PATCH 1/2] percpu: km: remove SMP check

[Dennis Zhou <dennis@kernel.org>]

On Tue, Feb 26, 2019 at 03:16:44PM +0000, Christopher Lameter wrote:
> On Mon, 25 Feb 2019, Dennis Zhou wrote:
> 
> > > @@ -27,7 +27,7 @@
> > >   *   chunk size is not aligned.  percpu-km code will whine about it.
> > >   */
> > >
> > > -#if defined(CONFIG_SMP) && defined(CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK)
> > > +#if defined(CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK)
> > >  #error "contiguous percpu allocation is incompatible with paged first chunk"
> > >  #endif
> > >
> > > --
> > > 2.16.4
> > >
> >
> > Hi,
> >
> > I think keeping CONFIG_SMP makes this easier to remember dependencies
> > rather than having to dig into the config. So this is a NACK from me.
> 
> But it simplifies the code and makes it easier to read.
> 
> 

I think the check isn't quite right after looking at it a little longer.
Looking at x86, I believe you can compile it with !SMP and
CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK will still be set. This should
still work because x86 has an MMU.

I think more correctly it would be something like below, but I don't
have the time to fully verify it right now.

Thanks,
Dennis

---
diff --git a/mm/percpu-km.c b/mm/percpu-km.c
index 0f643dc2dc65..69ccad7d9807 100644
--- a/mm/percpu-km.c
+++ b/mm/percpu-km.c
@@ -27,7 +27,7 @@
  *   chunk size is not aligned.  percpu-km code will whine about it.
  */
 
-#if defined(CONFIG_SMP) && defined(CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK)
+#if !defined(CONFIG_MMU) && defined(CONFIG_NEED_PER_CPU_PAGE_FIRST_CHUNK)
 #error "contiguous percpu allocation is incompatible with paged first chunk"
 #endif