Re: [RFC PATCH 2/7] mm: use rcu_ptr instead of rcu_head

[Harry Yoo <harry.yoo@oracle.com>]

On Mon, Feb 09, 2026 at 11:41:17AM +0100, Uladzislau Rezki wrote:
> On Fri, Feb 06, 2026 at 06:34:05PM +0900, Harry Yoo wrote:
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index 41dd01e8430c..89c781dcab58 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -2596,7 +2596,7 @@ struct vmap_block {
> >  	DECLARE_BITMAP(used_map, VMAP_BBMAP_BITS);
> >  	unsigned long dirty_min, dirty_max; /*< dirty range */
> >  	struct list_head free_list;
> > -	struct rcu_head rcu_head;
> > +	struct rcu_ptr rcu;
> >  	struct list_head purge;
> >  	unsigned int cpu;
> >  };
> >
> Why this change is needed?
> 
> If you want to save 8 bytes of vmap_block structure,

To be honest, because I didn't want to post a series with a feature
that has no users :)

The feature itself was requested by Alexei, because he doesn't
want to bump additional 8 bytes for each object in bpf side to use
kfree_rcu().

But not being familiar with kernel/bpf/,
I just added a few users in mm/ ;)

> then i do not see a big gain here.
> We do not have so many vmap_block objects.

But I agree that replacing existing users just because we can is not
an effective use of our time. I'll drop patch 2 in the next version
as it doesn't (or can't) demonstrate its benefit.

Are there any potential users that might benefit from this (other than bpf)?
don't know, but it will be interesting to explore.

> --
> Uladzislau Rezki

-- 
Cheers,
Harry / Hyeonggon