Re: [PATCH v3 04/19] mm: slub: implement SLUB version of obj_to_index()

[Christopher Lameter]

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@xxxxxx/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?