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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20200430171558.GA339283@carbon.dhcp.thefacebook.com>
Date:   Thu, 30 Apr 2020 10:15:58 -0700
From:   Roman Gushchin <guro@...com>
To:     Christopher Lameter <cl@...ux.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 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!