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

[Christopher Lameter]

On Mon, 27 Apr 2020, Roman Gushchin wrote:

> > 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?
> >
>
> So, the problem is that not all slab objects are accounted, and sometimes
> we don't know if advance if they are accounted or not (with the current semantics
> of __GFP_ACCOUNT and SLAB_ACCOUNT flags). So we either have to increase
> the size of ALL slab objects, either create a pair of slab caches for each size.

>
> The first option is not that cheap in terms of the memory overhead. Especially
> for those who disable cgroups using a boot-time option.


If the cgroups are disabled on boot time then you can switch back to the
compact version. Otherwise just add a pointer to each object. It will make
it consistent and there is not much memory wastage.

The problem comes about with the power of 2 caches in the kmalloc array.
If one keeps the "natural alignment" instead of going for the normal
alignment of slab caches then the alignment will cause a lot of memory
wastage and thus the scheme of off slab metadata is likely going to be
unavoidable.

But I think we are just stacking one bad idea onto another here making
things much more complex than they could be. Well at least this justifies
all our jobs .... (not mine I am out of work... hehehe)