Re: [PATCH v2] slab: replace cache_from_obj() with inline checks

[Eric Dumazet <edumazet@google.com>]

On Wed, Jan 21, 2026 at 7:57 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> 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 outdated comment in build_detached_freelist() reminds us.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>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Acked-by: Eric Dumazet <edumazet@google.com>

Thanks !