linux-kernel - Re: [PATCH 1/1] mm/slab: fix 'variable obj

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Date: Sun, 30 Jun 2024 12:20:42 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Vlastimil Babka <vbabka@...e.cz>
Cc: akpm@...ux-foundation.org, kent.overstreet@...ux.dev, 
	pasha.tatashin@...een.com, souravpanda@...gle.com, keescook@...omium.org, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	kernel test robot <lkp@...el.com>
Subject: Re: [PATCH 1/1] mm/slab: fix 'variable obj_exts set but not used' warning

On Mon, Jun 17, 2024 at 3:04 AM Vlastimil Babka <vbabka@...e.cz> wrote:
>
> On 6/15/24 12:59 AM, Suren Baghdasaryan wrote:
> > slab_post_alloc_hook() uses prepare_slab_obj_exts_hook() to obtain
> > slabobj_ext object. Currently the only user of slabobj_ext object in
> > this path is memory allocation profiling, therefore when it's not enabled
> > this object is not needed. This also generates a warning when compiling
> > with CONFIG_MEM_ALLOC_PROFILING=n. Move the code under this configuration
> > to fix the warning. If more slabobj_ext users appear in the future, the
> > code will have to be changed back to call prepare_slab_obj_exts_hook().
> >
> > Fixes: 4b8736964640 ("mm/slab: add allocation accounting into slab allocation and free paths")
> > Reported-by: kernel test robot <lkp@...el.com>
> > Closes: https://lore.kernel.org/oe-kbuild-all/202406150444.F6neSaiy-lkp@intel.com/
> > Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>
>
> Acked-by: Vlastimil Babka <vbabka@...e.cz>
>
> But it seems to me we could remove the whole #ifdef if current->alloc_tag
> (which doesn't exist with !MEM_ALLOC_PROFILING) had an access helper, or
> there was a alloc_tag_add_current() variant?

Hmm. I'll check if current->alloc_tag is the only reason for this
ifdef. If so then you are correct and we can simplify this code.

>
> > ---
> >  mm/slub.c | 7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/slub.c b/mm/slub.c
> > index 1373ac365a46..4927edec6a8c 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -3902,7 +3902,6 @@ bool slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru,
> >                         unsigned int orig_size)
> >  {
> >       unsigned int zero_size = s->object_size;
> > -     struct slabobj_ext *obj_exts;
> >       bool kasan_init = init;
> >       size_t i;
> >       gfp_t init_flags = flags & gfp_allowed_mask;
> > @@ -3945,9 +3944,11 @@ bool slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru,
> >               kmemleak_alloc_recursive(p[i], s->object_size, 1,
> >                                        s->flags, init_flags);
> >               kmsan_slab_alloc(s, p[i], init_flags);
> > +#ifdef CONFIG_MEM_ALLOC_PROFILING
> >               if (need_slab_obj_ext()) {
> > +                     struct slabobj_ext *obj_exts;
> > +
> >                       obj_exts = prepare_slab_obj_exts_hook(s, flags, p[i]);
> > -#ifdef CONFIG_MEM_ALLOC_PROFILING
> >                       /*
> >                        * Currently obj_exts is used only for allocation profiling.
> >                        * If other users appear then mem_alloc_profiling_enabled()
> > @@ -3955,8 +3956,8 @@ bool slab_post_alloc_hook(struct kmem_cache *s, struct list_lru *lru,
> >                        */
> >                       if (likely(obj_exts))
> >                               alloc_tag_add(&obj_exts->ref, current->alloc_tag, s->size);
> > -#endif
> >               }
> > +#endif
> >       }
> >
> >       return memcg_slab_post_alloc_hook(s, lru, flags, size, p);
> >
> > base-commit: c286c21ff94252f778515b21b6bebe749454a852
>