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!
>
