Re: [RFC PATCH V3 3/7] mm/slab: abstract slabobj_ext access via new slab_obj_ext() helper

[Suren Baghdasaryan]

On Wed, Oct 29, 2025 at 1:49 AM Harry Yoo <harry.yoo@xxxxxxxxxx> 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@xxxxxxxxxx> 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