slab: replace cache_from_obj() with inline checks

slab: replace cache_from_obj() with inline checks

Eric Dumazet has noticed cache_from_obj() is not inlined with clang and
suggested splitting it into two functions, where the smaller inlined one
assumes the fastpath is !CONFIG_SLAB_FREELIST_HARDENED. However most
distros enable it these days and so this would likely add a function
call to the object free fastpaths.

Instead take a step back and consider that cache_from_obj() is a relict
from when memcgs created their separate kmem_cache copies, as the
outdated comment in build_detached_freelist() reminds us.

Meanwhile hardening/debugging had reused cache_from_obj() to validate
that the freed object really belongs to a slab from the cache we think
we are freeing from.

In build_detached_freelist() simply remove this, because it did not
handle the NULL result from cache_from_obj() failure properly, nor
validate objects (for the NULL slab->slab_cache pointer) when called via
kfree_bulk(). If anyone is motivated to implement it properly, it should
be possible in a similar way to kmem_cache_free().

In kmem_cache_free(), do the hardening/debugging checks directly so they
are inlined by definition and virt_to_slab(obj) is performed just once.
In case they failed, call a newly introduced warn_free_bad_obj() that
performs the warnings outside of the fastpath, and leak the object.

As an intentional change, leak the object when slab->slab_cache differs
from the cache given to kmem_cache_free(). Previously we would only leak
when the object is not in a valid slab page or the slab->slab_cache
pointer is NULL, and otherwise trust the slab->slab_cache over the
kmem_cache_free() argument. But if those differ, it means something went
wrong enough that it's best not to continue freeing.

As a result the fastpath should be inlined in all configs and the
warnings are moved away.

Reported-by: Eric Dumazet <edumazet@google.com>
Closes: https://lore.kernel.org/all/20260115130642.3419324-1-edumazet@google.com/
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
Reviewed-by: Hao Li <hao.li@linux.dev>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>