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

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Message-ID: <20200515221255.GE94522@carbon.DHCP.thefacebook.com>
Date:   Fri, 15 May 2020 15:12:55 -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 Fri, May 15, 2020 at 09:45:30PM +0000, Christoph Lameter wrote:
> On Tue, 12 May 2020, Roman Gushchin wrote:
> 
> > > Add it to the metadata at the end of the object. Like the debugging
> > > information or the pointer for RCU freeing.
> >
> > Enabling debugging metadata currently disables the cache merging.
> > I doubt that it's acceptable to sacrifice the cache merging in order
> > to embed the memcg pointer?
> 
> Well then keep the merging even if you have a memcg pointer.
> 
> The disabling for debugging is only to simplify debugging. You dont have
> to deal with multiple caches actually using the same storage structures.
> 
> > Figuring out all these details will likely take several weeks, so the whole
> > thing will be delayed for one-two major releases (in the best case). Given that
> > the current implementation saves ~40% of slab memory, I think there is some value
> > in delivering it as it is. So I wonder if the idea of embedding the pointer
> > should be considered a blocker, or it can be implemented of top of the proposed
> > code (given it's not a user-facing api or something like this)?
> 
> Sorry no idea from my end here.

Ok, then I'll continue working on the embedding the pointer as an enhancement
*on top* of the current patchset. As I showed in my other e-mail, switching
to a different way of obj_cgroup storage is fairly trivial and doesn't change
much in the rest of the patchset.

Please, let me know if you're not ok with it.

Thanks!