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

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Date:   Fri, 15 May 2020 21:45:30 +0000 (UTC)
From:   Christopher Lameter <cl@...ux.com>
To:     Roman Gushchin <guro@...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 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.