Re: [PATCH 9/9] mm, slab/slub: move and improve cache_from_obj()

[Roman Gushchin]

On Wed, Jun 10, 2020 at 06:31:35PM +0200, Vlastimil Babka wrote:
> The function cache_from_obj() was added by commit b9ce5ef49f00 ("sl[au]b:
> always get the cache from its page in kmem_cache_free()") to support kmemcg,
> where per-memcg cache can be different from the root one, so we can't use
> the kmem_cache pointer given to kmem_cache_free().
> 
> Prior to that commit, SLUB already had debugging check+warning that could be
> enabled to compare the given kmem_cache pointer to one referenced by the slab
> page where the object-to-be-freed resides. This check was moved to
> cache_from_obj(). Later the check was also enabled for SLAB_FREELIST_HARDENED
> configs by commit 598a0717a816 ("mm/slab: validate cache membership under
> freelist hardening").
> 
> These checks and warnings can be useful especially for the debugging, which can
> be improved. Commit 598a0717a816 changed the pr_err() with WARN_ON_ONCE() to
> WARN_ONCE() so only the first hit is now reported, others are silent. This
> patch changes it to WARN() so that all errors are reported.
> 
> It's also useful to print SLUB allocation/free tracking info for the offending
> object, if tracking is enabled. We could export the SLUB print_tracking()
> function and provide an empty one for SLAB, or realize that both the debugging
> and hardening cases in cache_from_obj() are only supported by SLUB anyway. So
> this patch moves cache_from_obj() from slab.h to separate instances in slab.c
> and slub.c, where the SLAB version only does the kmemcg lookup and even could
> be completely removed once the kmemcg rework [1] is merged. The SLUB version
> can thus easily use the print_tracking() function. It can also use the
> kmem_cache_debug_flags() static key check for improved performance in kernels
> without the hardening and with debugging not enabled on boot.
> 
> [1] https://lore.kernel.org/r/20200608230654.828134-18-guro@xxxxxx
> 
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---
>  mm/slab.c |  8 ++++++++
>  mm/slab.h | 23 -----------------------
>  mm/slub.c | 21 +++++++++++++++++++++
>  3 files changed, 29 insertions(+), 23 deletions(-)
> 
> diff --git a/mm/slab.c b/mm/slab.c
> index 9350062ffc1a..6134c4c36d4c 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -3672,6 +3672,14 @@ void *__kmalloc_track_caller(size_t size, gfp_t flags, unsigned long caller)
>  }
>  EXPORT_SYMBOL(__kmalloc_track_caller);
>  
> +static inline struct kmem_cache *cache_from_obj(struct kmem_cache *s, void *x)
> +{
> +	if (memcg_kmem_enabled())
> +		return virt_to_cache(x);
> +	else
> +		return s;
> +}

Hm, it looks like all the SLAB version doesn't perform any sanity checks anymore.
Is it intended?

Also, Is it ever possible that s != virt_to_cache(x) if there are no bugs?

kmem_cache_free_bulk() in slab.c does contain the following:
	if (!orig_s) /* called via kfree_bulk */
		s = virt_to_cache(objp);
	else
		s = cache_from_obj(orig_s, objp);
which looks a bit strange with the version above.

> +
>  /**
>   * kmem_cache_free - Deallocate an object
>   * @cachep: The cache the allocation was from.
> diff --git a/mm/slab.h b/mm/slab.h
> index 815e4e9a94cd..c0c4244f75da 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -503,29 +503,6 @@ static __always_inline void uncharge_slab_page(struct page *page, int order,
>  	memcg_uncharge_slab(page, order, s);
>  }
>  
> -static inline struct kmem_cache *cache_from_obj(struct kmem_cache *s, void *x)
> -{
> -	struct kmem_cache *cachep;
> -
> -	/*
> -	 * When kmemcg is not being used, both assignments should return the
> -	 * same value. but we don't want to pay the assignment price in that
> -	 * case. If it is not compiled in, the compiler should be smart enough
> -	 * to not do even the assignment. In that case, slab_equal_or_root
> -	 * will also be a constant.
> -	 */
> -	if (!memcg_kmem_enabled() &&
> -	    !IS_ENABLED(CONFIG_SLAB_FREELIST_HARDENED) &&
> -	    !unlikely(s->flags & SLAB_CONSISTENCY_CHECKS))
> -		return s;
> -
> -	cachep = virt_to_cache(x);
> -	WARN_ONCE(cachep && !slab_equal_or_root(cachep, s),
> -		  "%s: Wrong slab cache. %s but object is from %s\n",
> -		  __func__, s->name, cachep->name);
> -	return cachep;
> -}
> -
>  static inline size_t slab_ksize(const struct kmem_cache *s)
>  {
>  #ifndef CONFIG_SLUB
> diff --git a/mm/slub.c b/mm/slub.c
> index efb08f2e9c66..f7a1d8537674 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -1524,6 +1524,10 @@ static bool freelist_corrupted(struct kmem_cache *s, struct page *page,
>  {
>  	return false;
>  }
> +
> +static void print_tracking(struct kmem_cache *s, void *object)
> +{
> +}
>  #endif /* CONFIG_SLUB_DEBUG */
>  
>  /*
> @@ -3175,6 +3179,23 @@ void ___cache_free(struct kmem_cache *cache, void *x, unsigned long addr)
>  }
>  #endif
>  
> +static inline struct kmem_cache *cache_from_obj(struct kmem_cache *s, void *x)
> +{
> +	struct kmem_cache *cachep;
> +
> +	if (!IS_ENABLED(CONFIG_SLAB_FREELIST_HARDENED) &&
> +	    !memcg_kmem_enabled() &&
> +	    !kmem_cache_debug_flags(s, SLAB_CONSISTENCY_CHECKS))
> +		return s;
> +
> +	cachep = virt_to_cache(x);
> +	if (WARN(cachep && !slab_equal_or_root(cachep, s),
> +		  "%s: Wrong slab cache. %s but object is from %s\n",
> +		  __func__, s->name, cachep->name))
> +		print_tracking(cachep, x);
> +	return cachep;
> +}

Maybe we can define a trivial SLAB version of kmem_cache_debug_flags()
and keep a single version of cache_from_obj()?

Thank you!