[PATCH] mm: percpu: fix section mismatch warning

[Arnd Bergmann]

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@xxxxxxxx>

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