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

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Message-ID: <CAJuCfpGFPuoUceB7SvAJPtVvzOOCzqS50yCcjbuMxV2a0e0KWA@mail.gmail.com>
Date: Wed, 29 Oct 2025 08:24:35 -0700
From: Suren Baghdasaryan <surenb@...gle.com>
To: Harry Yoo <harry.yoo@...cle.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 Wed, Oct 29, 2025 at 1:49 AM Harry Yoo <harry.yoo@...cle.com> wrote:
>
> 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().

True. I think we use slab->obj_exts to loop over its elements only in
two places: __memcg_slab_post_alloc_hook() and
__memcg_slab_free_hook(). I guess we could implement some kind of
slab_objext_foreach() construct to loop over all elements of
slab->obj_exts?

>
> --
> Cheers,
> Harry / Hyeonggon