Re: [PATCH v3 06/13] mm/slab: abstract slabobj_ext.ref access

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 7/28/26 15:41, Harry Yoo wrote:
> On Mon, Jul 27, 2026 at 02:54:00PM +0200, Vlastimil Babka (SUSE) wrote:
>> In preparation for changes to the structure, abstract access to the ref
>> field with a slab_obj_ext_codetag_ref() function. Rename the field to
>> _ctref to make an unexpected direct access a compile error.
>> 
>> No functional change intended.
>> 
>> Reviewed-by: Suren Baghdasaryan <surenb@google.com>
>> Reviewed-by: Hao Li <hao.li@linux.dev>
>> Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
>> ---
> 
> Looks good to me,
> Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>
> 
> With a suggestion: it would be nice to have debug warnings
> that trigger when slab_obj_ext_codetag_ref() is compiled but
> not supposed to be called.
> 
> e.g.) for kfence objects, or later in the series when we
> reduce slabobj_ext memory (!slab_obj_ext_has_codetag()) 
> 
> Similarly, it would be nice (later in the series) to have a warning
> in slab_obj_ext_objcg() when slab_needs_objcg() returns false.

OK good idea but we can't do it right now as the helpers are introduced
later.

So for 9/13 mm/slab: introduce slab_obj_ext_has_codetag():

diff --git a/mm/slab.h b/mm/slab.h
index 8f352d9f4d91..fbad99da093c 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -726,6 +726,8 @@ static inline void slab_obj_ext_set_objcg(struct slabobj_ext *obj_ext,
 static inline union codetag_ref *
 slab_obj_ext_codetag_ref(struct slab *slab, struct slabobj_ext *obj_ext)
 {
+       VM_WARN_ON_ONCE(!slab_obj_ext_has_codetag());
+
        if (IS_ENABLED(CONFIG_MEMCG))
                obj_ext += 1;
 
... and for objcg we would need to start passing slab pointer to
slab_obj_ext_objcg() and slab_obj_ext_set_objcg(). Hmm...


>>  mm/slab.h | 10 +++++++++-
>>  mm/slub.c | 42 ++++++++++++++++++++++++++++--------------
>>  2 files changed, 37 insertions(+), 15 deletions(-)