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

[Suren Baghdasaryan <surenb@google.com>]

On Wed, Oct 29, 2025 at 6:26 PM Harry Yoo <harry.yoo@oracle.com> wrote:
>
> On Wed, Oct 29, 2025 at 08:24:35AM -0700, Suren Baghdasaryan wrote:
> > On Wed, Oct 29, 2025 at 1:49 AM Harry Yoo <harry.yoo@oracle.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@oracle.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?
>
> Not sure if that would help here. In __memcg_slab_free_hook() we want to
> iterate only some of (not all of) elements from the same slab
> (we know they're from the same slab as we build detached freelist and
> sort the array) and so we read slab->obj_exts only once.
>
> In __memcg_slab_post_alloc_hook() we don't know if the objects are from
> the same slab, so we read slab->obj_exts multiple times and charge them.
>
> I think we need to either 1) remove slab_obj_exts() and
> then introduce is_slab_obj_exts() and see if it has impact on
> performance, or 2) keep it as-is.

Ok, it sounds like too much effort for avoiding a direct accessor.
Let's go with (2) for now.

>
> Thanks!
>
> --
> Cheers,
> Harry / Hyeonggon