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

[Roman Gushchin]

On Thu, Apr 30, 2020 at 04:29:50PM +0000, Christoph Lameter wrote:
> 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.

It's a very good point, and it's an argument to stick with the current design
(an external vector of memcg pointers).

> 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)

Sorry, but what exactly do you mean?
I don't think reducing the kernel memory footprint by almost half
is such a bad idea.

Thanks!