linux-kernel - Re: [PATCH v3 04/19] mm: slub: implement SLUB version of obj_to

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Date:   Mon, 27 Apr 2020 16:21:01 +0000 (UTC)
From:   Christopher Lameter <cl@...ux.com>
To:     Roman Gushchin <guro@...com>
cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>, linux-mm@...ck.org,
        kernel-team@...com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 04/19] mm: slub: implement SLUB version of
 obj_to_index()

On Fri, 24 Apr 2020, Roman Gushchin wrote:

> > The patch seems to only use it for setup and debugging? It is used for
> > every "accounted" allocation???? Where? And what is an "accounted"
> > allocation?
> >
> >
>
> Please, take a look at the whole series:
> https://lore.kernel.org/linux-mm/20200422204708.2176080-1-guro@fb.com/T/#t
>
> I'm sorry, I had to cc you directly for the whole thing. Your feedback
> will be highly appreciated.
>
> It's used to calculate the offset of the memcg pointer for every slab
> object which is charged to a memory cgroup. So it must be quite hot.


Ahh... Thanks. I just looked at it.

You need this because you have a separate structure attached to a page
that tracks membership of the slab object to the cgroup. This is used to
calculate the offset into that array....

Why do you need this? Just slap a pointer to the cgroup as additional
metadata onto the slab object. Is that not much simpler, safer and faster?