Re: [PATCH] mm/percpu: account full object size for percpu memory allocations

[Harry Yoo <harry@kernel.org>]

On Wed, Oct 14, 2026 at 09:43:48AM -0700, Joshua Hahn wrote:
> On Tue, 13 Oct 2026 20:34:39 +0100 "Harry Yoo (Meta)" <harry@kernel.org> wrote:
> Hi Harry,

Hi Joshua,

> This looks quite good to me. Thanks for finding this!
> Reviewed-by: Joshua Hahn <joshua.hahnjy@gmail.com>

Thanks for reviewing!

> > diff --git a/mm/percpu.c b/mm/percpu.c
> > index 3eff382e565a..d7883507dc15 100644
> > --- a/mm/percpu.c
> > +++ b/mm/percpu.c
> > @@ -1696,14 +1696,15 @@ static void pcpu_alloc_tag_alloc_hook(struct pcpu_chunk *chunk, int off,
> >  {
> >  	if (mem_alloc_profiling_enabled() && likely(chunk->obj_exts)) {
> >  		alloc_tag_add(&chunk->obj_exts[off >> PCPU_MIN_ALLOC_SHIFT].tag,
> > -			      current->alloc_tag, size);
> > +			      current->alloc_tag, pcpu_obj_full_size(size));
> >  	}
> >  }
> 
> So @size by itself isn't used, so I was looking to see if we could just
> pass pcpu_obj_full_size(size) to the function instead. Looking at the
> call site, pcpu_memcg_post_alloc_hook() also has the same oddity where
> pcpu_obj_full_size(size) is used for all calcualtions.

Agreed.

At first look I actually tried to clean up things, then realized
I need to backport this and so cleanup should be a follow-up :) 

> So I'm wondering if we can send a follow up to just unify all of these
> and pass pcpu_obj_full_size(size) where the caller never uses @size
> by itself, I think it would also help in demonstrating the intent to
> account the full size. Same idea for the free hook below as well,
> paired with pcpu_memcg_free_hook().

Makes perfect sense to me.

Perhaps mm/percpu.c should still distinguish 'full_size' (uh, naming
sucks) and 'size'.

> Happy to follow up on that myself too : -)

Sure, thanks!
Happy to review that from my side.

> Anyways, the change looks good. Just wanted to bring this up as a
> follow-up cleanup patch.

Thanks!

> Have a great day Harry!

Likewise, have a nice day, Joshua.

-- 
Cheers,
Harry / Hyeonggon