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

[David Rientjes <rientjes@google.com>]

On Wed, 21 Jan 2026, Eric Dumazet 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 outdated comment in build_detached_freelist() reminds us.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>
> 

Tested-by: David Rientjes <rientjes@google.com>