Note no effort is made to make sure structs embedding the namespace are
themselves aligned, so this is not guaranteed to eliminate cacheline
bouncing due to refcount management.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
---
 include/linux/ns/ns_common_types.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/linux/ns/ns_common_types.h b/include/linux/ns/ns_common_types.h
index b332b019b29c..0014fbc1c626 100644
--- a/include/linux/ns/ns_common_types.h
+++ b/include/linux/ns/ns_common_types.h
@@ -108,11 +108,13 @@ extern const struct proc_ns_operations utsns_operations;
  * @ns_tree: namespace tree nodes and active reference count
  */
 struct ns_common {
+	struct {
+		refcount_t __ns_ref; /* do not use directly */
+	} ____cacheline_aligned_in_smp;
 	u32 ns_type;
 	struct dentry *stashed;
 	const struct proc_ns_operations *ops;
 	unsigned int inum;
-	refcount_t __ns_ref; /* do not use directly */
 	union {
 		struct ns_tree;
 		struct rcu_head ns_rcu;
-- 
2.48.1

On 11/23, Mateusz Guzik wrote:
>
>  struct ns_common {
> +	struct {
> +		refcount_t __ns_ref; /* do not use directly */
> +	} ____cacheline_aligned_in_smp;

Cough... stupid question. Why not just

	refcount_t __ns_ref ____cacheline_aligned_in_smp;

? why do we need the anonymous struct?

Oleg.

On Sun, Nov 23, 2025 at 7:58 PM Oleg Nesterov <oleg@redhat.com> wrote:
>
> On 11/23, Mateusz Guzik wrote:
> >
> >  struct ns_common {
> > +     struct {
> > +             refcount_t __ns_ref; /* do not use directly */
> > +     } ____cacheline_aligned_in_smp;
>
> Cough... stupid question. Why not just
>
>         refcount_t __ns_ref ____cacheline_aligned_in_smp;
>
> ? why do we need the anonymous struct?
>

This would merely align the offset of the field, with the rest
directly following.

On 11/23, Mateusz Guzik wrote:
>
> On Sun, Nov 23, 2025 at 7:58 PM Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > On 11/23, Mateusz Guzik wrote:
> > >
> > >  struct ns_common {
> > > +     struct {
> > > +             refcount_t __ns_ref; /* do not use directly */
> > > +     } ____cacheline_aligned_in_smp;
> >
> > Cough... stupid question. Why not just
> >
> >         refcount_t __ns_ref ____cacheline_aligned_in_smp;
> >
> > ? why do we need the anonymous struct?
> >
>
> This would merely align the offset of the field, with the rest
> directly following.

Ah. I didn't bother to read the changelog and misunderstood the intent.

OK, thanks. At least I have warned you that my question is stupid ;)

Oleg.