Re: [PATCH v3 08/13] mm/slab: change struct slabobj_ext to a union

[Harry Yoo <harry@kernel.org>]

On Mon, Jul 27, 2026 at 02:54:02PM +0200, Vlastimil Babka (SUSE) wrote:
> Currently, struct slabobj_ext can hold both objcg pointer and
> codetag_ref (when both are compile-enabled) and there is an array of as
> many slabobj_ext instances as there are objects in a slab.
> 
> This makes the layout fixed so even if codetag_ref is unused (because
> memory allocation profiling is disabled), the space for them is
> allocated and wasted. Similarly, some caches (currently kmalloc_normal)
> do not ever need objcg pointers, leading to wasted memory with memory
> allocation profiling enabled.
> 
> To make this more flexible, change the layout so that struct slabobj_ext
> becomes a union of objcg pointer and codetag_ref (to ensure uniform
> size; in practice both are the same size anyway). The slabobj_ext array
> then can have twice as many elements as before. For cache locality
> purposes, the effective memory layout is unchanged, so objcg and codetag
> ref for a given object are still adjacent.
> 
> cache_obj_ext_size() returns the effective size of (0-2) struct
> slabobj_ext's for a cache, slab_obj_ext_size() for a slab. Currently
> both return a constant value derived from the config options, but will
> be made dynamic later. Replace all sizeof(slabobj_ext) usage with these.
> 
> No functional change intended, the layout is still effectively static.
> 
> Reviewed-by: Suren Baghdasaryan <surenb@google.com>
> Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
> ---

Looks good to me,
Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>