linux-ext4 - Re: [RFC PATCH V3 3/7] mm/slab: abstract slabobj_ext access via new slab_obj

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Message-ID: <aQHVB_8NVMZ2cuvh@hyeyoo>
Date: Wed, 29 Oct 2025 17:49:30 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: Suren Baghdasaryan <surenb@...gle.com>
Cc: akpm@...ux-foundation.org, vbabka@...e.cz, andreyknvl@...il.com,
        cl@...ux.com, dvyukov@...gle.com, glider@...gle.com,
        hannes@...xchg.org, linux-mm@...ck.org, mhocko@...nel.org,
        muchun.song@...ux.dev, rientjes@...gle.com, roman.gushchin@...ux.dev,
        ryabinin.a.a@...il.com, shakeel.butt@...ux.dev,
        vincenzo.frascino@....com, yeoreum.yun@....com, tytso@....edu,
        adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH V3 3/7] mm/slab: abstract slabobj_ext access via new
 slab_obj_ext() helper

On Tue, Oct 28, 2025 at 10:55:39AM -0700, Suren Baghdasaryan wrote:
> On Mon, Oct 27, 2025 at 5:29 AM Harry Yoo <harry.yoo@...cle.com> wrote:
> >
> > Currently, the slab allocator assumes that slab->obj_exts is a pointer
> > to an array of struct slabobj_ext objects. However, to support storage
> > methods where struct slabobj_ext is embedded within objects, the slab
> > allocator should not make this assumption. Instead of directly
> > dereferencing the slabobj_exts array, abstract access to
> > struct slabobj_ext via helper functions.
> >
> > Introduce a new API slabobj_ext metadata access:
> >
> >   slab_obj_ext(slab, obj_exts, index) - returns the pointer to
> >   struct slabobj_ext element at the given index.
> >
> > Directly dereferencing the return value of slab_obj_exts() is no longer
> > allowed. Instead, slab_obj_ext() must always be used to access
> > individual struct slabobj_ext objects.
> 
> If direct access to the vector is not allowed, it would be better to
> eliminate slab_obj_exts() function completely and use the new
> slab_obj_ext() instead. I think that's possible. We might need an
> additional `bool is_slab_obj_exts()` helper for an early check before
> we calculate the object index but that's quite easy.

Good point, but that way we cannot avoid reading slab->obj_exts
multiple times when we access slabobj_ext of multiple objects
as it's accessed via READ_ONCE().

-- 
Cheers,
Harry / Hyeonggon