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

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Message-ID: <20180314084329.y7735ecw2is5i5pd@esperanza>
Date:   Wed, 14 Mar 2018 11:43:29 +0300
From:   Vladimir Davydov <vdavydov.dev@...il.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Christopher Lameter <cl@...ux.com>,
        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 10:36:52AM -0700, Shakeel Butt wrote:
> 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.

This was a mistake - I missed that __kmem_cache_create() overwrites
kmem_cache->size. Thanks for fixing this.