linux-kernel - Re: [PATCH] slab, slub: remove size disparity on debug kernel

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Date:   Tue, 13 Mar 2018 10:36:52 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Christopher Lameter <cl@...ux.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>
Cc:     Suleiman Souhlal <suleiman@...gle.com>,
        Greg Thelen <gthelen@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] slab, slub: remove size disparity on debug kernel

On Tue, Mar 13, 2018 at 10:19 AM, Christopher Lameter <cl@...ux.com> wrote:
> On Tue, 13 Mar 2018, Shakeel Butt wrote:
>
>> However for SLUB in debug kernel, the sizes were same. On further
>> inspection it is found that SLUB always use kmem_cache.object_size to
>> measure the kmem_cache.size while SLAB use the given kmem_cache.size. In
>> the debug kernel the slab's size can be larger than its object_size.
>> Thus in the creation of non-root slab, the SLAB uses the root's size as
>> base to calculate the non-root slab's size and thus non-root slab's size
>> can be larger than the root slab's size. For SLUB, the non-root slab's
>> size is measured based on the root's object_size and thus the size will
>> remain same for root and non-root slab.
>
> Note that the object_size and size may differ for SLUB based on kernel
> parameters and slab configuration. For SLAB these are compilation options.
>

Thanks for the explanation.

>> @@ -379,7 +379,7 @@ struct kmem_cache *find_mergeable(unsigned int size, unsigned int align,
>>  }
>>
>>  static struct kmem_cache *create_cache(const char *name,
>> -             unsigned int object_size, unsigned int size, unsigned int align,
>> +             unsigned int object_size, unsigned int align,
>>               slab_flags_t flags, unsigned int useroffset,
>
> Why was both the size and object_size passed during cache creation in the
> first place? From the flags etc the slab logic should be able to compute
> the actual bytes required for each object and its metadata.
>

+Vladimir

I think it was introduced by 794b1248be4e7 ("memcg, slab: separate
memcg vs root cache creation paths") but I could not find out the
reason.