mm/slab: change struct slabobj_ext to a union

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>
Link: https://patch.msgid.link/20260727-b4-objext_split-v3-8-c29ef0f1f257@kernel.org
Reviewed-by: Harry Yoo (Oracle) <harry@kernel.org>
Reviewed-by: Hao Li <hao.li@linux.dev>
Signed-off-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

0 files changed, 0 insertions, 0 deletions