linux-kernel - Re: [PATCH v4 04/19] mm: slub: implement SLUB version of obj_to

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Message-ID: <20200527194455.GB47905@cmpxchg.org>
Date:   Wed, 27 May 2020 15:44:55 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Roman Gushchin <guro@...com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Michal Hocko <mhocko@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>, linux-mm@...ck.org,
        Vlastimil Babka <vbabka@...e.cz>, kernel-team@...com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 04/19] mm: slub: implement SLUB version of
 obj_to_index()

On Tue, May 26, 2020 at 02:42:12PM -0700, Roman Gushchin wrote:
> This commit implements SLUB version of the obj_to_index() function,
> which will be required to calculate the offset of obj_cgroup in the
> obj_cgroups vector to store/obtain the objcg ownership data.
> 
> To make it faster, let's repeat the SLAB's trick introduced by
> commit 6a2d7a955d8d ("[PATCH] SLAB: use a multiply instead of a
> divide in obj_to_index()") and avoid an expensive division.
> 
> Vlastimil Babka noticed, that SLUB does have already a similar
> function called slab_index(), which is defined only if SLUB_DEBUG
> is enabled. The function does a similar math, but with a division,
> and it also takes a page address instead of a page pointer.
> 
> Let's remove slab_index() and replace it with the new helper
> __obj_to_index(), which takes a page address. obj_to_index()
> will be a simple wrapper taking a page pointer and passing
> page_address(page) into __obj_to_index().
> 
> Signed-off-by: Roman Gushchin <guro@...com>
> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>

Acked-by: Johannes Weiner <hannes@...xchg.org>