Re: [PATCH] percpu: fix clang modpost warning in pcpu_build_alloc_info()

[Arnd Bergmann <arnd@kernel.org>]

On Tue, Jan 5, 2021 at 1:55 AM Dennis Zhou <dennis@kernel.org> wrote:
>
> On Mon, Jan 04, 2021 at 04:46:51PM -0700, Nathan Chancellor wrote:
> > On Thu, Dec 31, 2020 at 09:28:52PM +0000, Dennis Zhou wrote:
> > >
>
> Hi Nathan,
>
> >
> > Hi Dennis,
> >
> > I did a bisect of the problematic config against defconfig and it points
> > out that CONFIG_GCOV_PROFILE_ALL is in the bad config but not the good
> > config, which makes some sense as that will mess with clang's inlining
> > heuristics. It does not appear to be the single config that makes a
> > difference but it gives some clarity.
> >
>
> Ah, thanks. To me it's kind of a corner case that I don't have a lot of
> insight into. __init code is pretty limited and this warning is really
> at the compilers whim. However, in this case only clang throws this
> warning.
>
> > I do not personally have any strong opinions around the patch but is it
> > really that much wasted memory to just annotate mask with __refdata?
>
> It's really not much memory, 1 bit per max # of cpus. The reported
> config is on the extreme side compiling with 8k NR_CPUS, so 1kb. I'm
> just not in love with the idea of adding a patch to improve readability
> and it cost idle memory to resolve a compile time warning.
>
> If no one else chimes in in the next few days, I'll probably just apply
> it and go from there. If another issue comes up I'll drop this and tag
> it as __refdata.

I've come across this one again in linux-next today, and found that
I had an old patch for it already, that I had never submitted:

From 7d6f40414490092b86f1a64d8c42426ee350da1a Mon Sep 17 00:00:00 2001
From: Arnd Bergmann <arnd@arndb.de>
Date: Mon, 7 Dec 2020 23:24:20 +0100
Subject: [PATCH] mm: percpu: fix section mismatch warning

Building with arm64 clang sometimes (fairly rarely) shows a
warning about the pcpu_build_alloc_info() function:

WARNING: modpost: vmlinux.o(.text+0x21697c): Section mismatch in
reference from the function cpumask_clear_cpu() to the variable
.init.data:pcpu_build_alloc_info.mask
The function cpumask_clear_cpu() references
the variable __initdata pcpu_build_alloc_info.mask.
This is often because cpumask_clear_cpu lacks a __initdata
annotation or the annotation of pcpu_build_alloc_info.mask is wrong.

What appears to be going on here is that the compiler decides to not
inline the cpumask_clear_cpu() function that is marked 'inline' but not
'always_inline', and it then produces a specialized version of it that
references the static mask unconditionally as an optimization.

Marking cpumask_clear_cpu() as __always_inline would fix it, as would
removing the __initdata annotation on the variable.  I went for marking
the function as __attribute__((flatten)) instead because all functions
called from it are really meant to be inlined here, and it prevents
the same problem happening here again. This is unlikely to be a problem
elsewhere because there are very few function-local static __initdata
variables in the kernel.

Fixes: 6c207504ae79 ("percpu: reduce the number of cpu distance comparisons")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

diff --git a/mm/percpu.c b/mm/percpu.c
index 5ede8dd407d5..527181c46b08 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -2662,10 +2662,9 @@ early_param("percpu_alloc", percpu_alloc_setup);
  * On success, pointer to the new allocation_info is returned.  On
  * failure, ERR_PTR value is returned.
  */
-static struct pcpu_alloc_info * __init pcpu_build_alloc_info(
-                               size_t reserved_size, size_t dyn_size,
-                               size_t atom_size,
-                               pcpu_fc_cpu_distance_fn_t cpu_distance_fn)
+static struct pcpu_alloc_info * __init __attribute__((flatten))
+pcpu_build_alloc_info(size_t reserved_size, size_t dyn_size, size_t atom_size,
+                     pcpu_fc_cpu_distance_fn_t cpu_distance_fn)
 {
        static int group_map[NR_CPUS] __initdata;
        static int group_cnt[NR_CPUS] __initdata;


Not sure if this would be any better than your patch.

       Arnd