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Date:   Wed, 28 Jun 2023 13:59:34 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Julian Pidancet <julian.pidancet@oracle.com>
Cc:     Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Hyeonggon Yoo <42.hyeyoo@gmail.com>, linux-mm@kvack.org,
        Jonathan Corbet <corbet@lwn.net>, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, Matthew Wilcox <willy@infradead.org>,
        Rafael Aquini <aquini@redhat.com>
Subject: Re: [PATCH] mm/slub: disable slab merging in the default
 configuration
Message-ID: <202306281358.E6E6C2759@keescook>
References: <20230627132131.214475-1-julian.pidancet@oracle.com>
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On Tue, Jun 27, 2023 at 03:21:31PM +0200, Julian Pidancet wrote:
> Make CONFIG_SLAB_MERGE_DEFAULT default to n unless CONFIG_SLUB_TINY is
> enabled. Benefits of slab merging is limited on systems that are not
> memory constrained: the overhead is negligible and evidence of its
> effect on cache hotness is hard to come by.
> 
> On the other hand, distinguishing allocations into different slabs will
> make attacks that rely on "heap spraying" more difficult to carry out
> with success.
> 
> Take sides with security in the default kernel configuration over
> questionnable performance benefits/memory efficiency.
> 
> Signed-off-by: Julian Pidancet <julian.pidancet@oracle.com>
> ---
> In an attempt to assess the performance impact of disabling slab
> merging, a timed linux kernel compilation test has been conducted first
> using slab_merge, then using slab_nomerge. Both tests started in an
> identical state.  Commodity hardware was used: a laptop with an AMD Ryzen
> 5 3500U CPU, and 16GiB of RAM. The kernel source files were placed on
> an XFS partition because of the extensive use of slab caches in XFS.
> 
> The results are as follows:
> 
>       | slab_merge       | slab_nomerge     |
> ------+------------------+------------------|
> Time  | 489.074 ? 10.334 | 489.975 ? 10.350 |
> Min   |          459.688 |          460.554 |
> Max   |          493.126 |          494.282 |
> 
> The benchmark favors the configuration where merging is disabled, but the
> difference is only ~0.18%, well under statistical significance.

As mentioned, please include these kinds of perf notes in the commit
log; it's useful to see later. :)

Regardless, yes, please. I have been running slab_nomerge on all my
systems for years and years now.

With the typo fixed and commit log updated, please consider this:

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

-- 
Kees Cook