linux-kernel - Re: [PATCH 9/9] mm, slab/slub: move and improve cache_from

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Date:   Thu, 11 Jun 2020 11:56:53 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Roman Gushchin <guro@...com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com,
        vinmenon@...eaurora.org, Kees Cook <keescook@...omium.org>,
        Matthew Garrett <mjg59@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Vijayanand Jitta <vjitta@...eaurora.org>
Subject: Re: [PATCH 9/9] mm, slab/slub: move and improve cache_from_obj()

On 6/11/20 12:46 AM, Roman Gushchin wrote:
> On Wed, Jun 10, 2020 at 06:31:35PM +0200, Vlastimil Babka wrote:
>> @@ -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?

Yes, it was the same before commit b9ce5ef49f00. The commit could have been more
precise - kmemcg needs virt_to_cache(), but not the sanity check. The SLUB
version also shouldn't really be doing the sanity check if only
memcg_kmem_enabled() is true (and not the debugging/hardening), but the code
then looks ugly and I hope this will just fix itself with your kmemcg slab rework.

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

Well, only in the kmemcg case it should be possible.

> 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.

Looks fine to me. If we are called with non-NULL s, and kmemcg is not enabled,
we can just trust s. If we are called with NULL s (via kfree_bulk()) we need to
get cache from the object, even if kmemcg is not enabled, so we do
virt_to_cache() unconditionally.
Once your series is fully accepted, we can remove SLAB's cache_from_obj() and
the whole 'else' part in the hunk above. Or am I missing something?

>> @@ -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()?

I think the result would be more obfuscated than just making it plain that SLAB
doesn't have those SLUB features. And I still hope SLAB's version will go away
completely. If your series is accepted first, then this patch based in that will
not introduce slab.c cache_from_obj() at all.

> Thank you!
>