From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 62814EB64DC for ; Tue, 27 Jun 2023 19:32:21 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229835AbjF0TcU (ORCPT ); Tue, 27 Jun 2023 15:32:20 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49590 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229777AbjF0TcU (ORCPT ); Tue, 27 Jun 2023 15:32:20 -0400 Received: from mail-pl1-x636.google.com (mail-pl1-x636.google.com [IPv6:2607:f8b0:4864:20::636]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id CE8ED19B5 for ; Tue, 27 Jun 2023 12:32:17 -0700 (PDT) Received: by mail-pl1-x636.google.com with SMTP id d9443c01a7336-1b82afd519fso31235ad.1 for ; Tue, 27 Jun 2023 12:32:17 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20221208; t=1687894337; x=1690486337; h=mime-version:references:message-id:in-reply-to:subject:cc:to:from :date:from:to:cc:subject:date:message-id:reply-to; bh=ck21h6YvUFdrodIGjj6YsU+OWYBVIflql5oiEYBppsM=; b=SLlivscbvsR1PVQf5GVSNkbJ2yyQ76xW2h8F30oXjhTUb4nMDmTr7nvn0j9cDaRm0r DhW92Y68Ihi9JRL8OJ1OG0HY/EwZjhPPUicsv4v7vsW4/BV38ukoNsM5XcclyqoYQZD7 Va67Z7ErZMtA2svIeftPoqkaPmQ6Vn7Y+kAXnricFTcFNAM119XeFqEETW9uJ/czqSai iuupc+e+x8ZLEbNLbxrxsX6vGXvwLiKoEoG9l4v5qWc2+pwYppsPVvsxACNDuPe8jhGu l9bSrQh2JzUdXPV71x3+YHM7N4DTRL8RJ/VTaGMeFQYsHP6y48oj3fyyEWKUfFxBfJtW viNw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20221208; t=1687894337; x=1690486337; h=mime-version:references:message-id:in-reply-to:subject:cc:to:from :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=ck21h6YvUFdrodIGjj6YsU+OWYBVIflql5oiEYBppsM=; b=ARbN/1cDqIU14+RJn+SiTPuqwOYbJXAHqVbYgIIVsbjHXk1uAuLfLrO+FfdZlo8CMB dwvriSp4QbYsow36aAZ6anaaYoqzoW4zl5C6rgtniQFrlOMMxkhfKy7AkQfkbHUJ85Fq mD8fg4ajDxwBC1GyTchRdlp9YtS+3q/mmZXb3yw/eUDsGmmSjamdLSKBJNAqrlD+x0vK +3BtMhhSGRvMXrDlWoQ1nMpqYxZrssvqizd91rQVm7cqo4PWwAF9rurXorb7+/kwZ7SA fXrL7IC9wrxuJmKoIGUFvEwrsmNvUussjTkATSUFAqgZciOiIpJWGdJ1zan3cqm/lzUg 0ITQ== X-Gm-Message-State: AC+VfDzeRVqn9qHHSrtjMj9769Yj9zv/XDyZTYMHsl1euz1hh8OPtEIW jlfdE2CsSY0KLUVzQdkAw9VsRg== X-Google-Smtp-Source: ACHHUZ6arFC51dXDUj/BzIkIsYYKV1kMOuQaZX0aowuibpY/Cz1H30KX128wSJWTPXqV6huFvB/TDw== X-Received: by 2002:a17:902:ec91:b0:1b0:ceb:d67f with SMTP id x17-20020a170902ec9100b001b00cebd67fmr33959plg.8.1687894337073; Tue, 27 Jun 2023 12:32:17 -0700 (PDT) Received: from [2620:0:1008:15:ab4c:7287:2273:1bd0] ([2620:0:1008:15:ab4c:7287:2273:1bd0]) by smtp.gmail.com with ESMTPSA id iw4-20020a170903044400b001b80104446fsm4192320plb.20.2023.06.27.12.32.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 27 Jun 2023 12:32:16 -0700 (PDT) Date: Tue, 27 Jun 2023 12:32:15 -0700 (PDT) From: David Rientjes To: Julian Pidancet cc: Christoph Lameter , Pekka Enberg , Joonsoo Kim , Andrew Morton , Vlastimil Babka , Roman Gushchin , Hyeonggon Yoo <42.hyeyoo@gmail.com>, linux-mm@kvack.org, Jonathan Corbet , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, Matthew Wilcox , Kees Cook , Rafael Aquini Subject: Re: [PATCH] mm/slub: disable slab merging in the default configuration In-Reply-To: <20230627132131.214475-1-julian.pidancet@oracle.com> Message-ID: <48bd9819-3571-6b53-f1ad-ec013be742c0@google.com> References: <20230627132131.214475-1-julian.pidancet@oracle.com> MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="2003067076-1055337799-1687894336=:2486373" Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org This message is in MIME format. The first part should be readable text, while the remaining parts are likely unreadable without MIME-aware tools. --2003067076-1055337799-1687894336=:2486373 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT On Tue, 27 Jun 2023, 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. > I don't have an objection to this, I think it makes sense. When you say overhead here, I assume you're referring to memory footprint? Did you happen to have some system-wide numbers for what that looks like when running some benchmarks, or even what the slab usage looks like after a fresh boot? > 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 > --- > 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. > I think this data should be in the changelog itself, as well as any numbers to share on the memory footprint differences. > .../admin-guide/kernel-parameters.txt | 29 ++++++++++--------- > Documentation/mm/slub.rst | 5 ++-- > mm/Kconfig | 6 ++-- > 3 files changed, 21 insertions(+), 19 deletions(-) > > diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt > index 9e5bab29685f..5fbf6ed3c62e 100644 > --- a/Documentation/admin-guide/kernel-parameters.txt > +++ b/Documentation/admin-guide/kernel-parameters.txt > @@ -5652,21 +5652,22 @@ > > slram= [HW,MTD] > > - slab_merge [MM] > - Enable merging of slabs with similar size when the > - kernel is built without CONFIG_SLAB_MERGE_DEFAULT. > - > slab_nomerge [MM] > - Disable merging of slabs with similar size. May be > - necessary if there is some reason to distinguish > - allocs to different slabs, especially in hardened > - environments where the risk of heap overflows and > - layout control by attackers can usually be > - frustrated by disabling merging. This will reduce > - most of the exposure of a heap attack to a single > - cache (risks via metadata attacks are mostly > - unchanged). Debug options disable merging on their > - own. > + Disable merging of slabs with similar size when > + the kernel is built with CONFIG_SLAB_MERGE_DEFAULT. > + Allocations of the same size made in distinct > + caches will be placed in separate slabs. In > + hardened environment, the risk of heap overflows > + and layout control by attackers can usually be > + frustrated by disabling merging. > + > + slab_merge [MM] > + Enable merging of slabs with similar size. May be > + necessary to reduce overhead or increase cache > + hotness of objects, at the cost of increased > + exposure in case of a heap attack to a single > + cache. (risks via metadata attacks are mostly > + unchanged). > For more information see Documentation/mm/slub.rst. > > slab_max_order= [MM, SLAB] > diff --git a/Documentation/mm/slub.rst b/Documentation/mm/slub.rst > index be75971532f5..e2549f4a98dd 100644 > --- a/Documentation/mm/slub.rst > +++ b/Documentation/mm/slub.rst > @@ -122,8 +122,9 @@ used on the wrong slab. > Slab merging > ============ > > -If no debug options are specified then SLUB may merge similar slabs together > -in order to reduce overhead and increase cache hotness of objects. > +If the kernel is built with ``CONFIG_SLAB_MEGE_DEFAULT`` or if ``slab_merge`` s/MEGE/MERGE/ > +is specified on the kernel command line, then SLUB may merge similar slabs > +together in order to reduce overhead and increase cache hotness of objects. Specify that this is memory overhead? > ``slabinfo -a`` displays which slabs were merged together. > > Slab validation > diff --git a/mm/Kconfig b/mm/Kconfig > index 7672a22647b4..05b0304302d4 100644 > --- a/mm/Kconfig > +++ b/mm/Kconfig > @@ -255,7 +255,7 @@ config SLUB_TINY > > config SLAB_MERGE_DEFAULT > bool "Allow slab caches to be merged" > - default y > + default n > depends on SLAB || SLUB > help > For reduced kernel memory fragmentation, slab caches can be > @@ -264,8 +264,8 @@ config SLAB_MERGE_DEFAULT > overwrite objects from merged caches (and more easily control > cache layout), which makes such heap attacks easier to exploit > by attackers. By keeping caches unmerged, these kinds of exploits > - can usually only damage objects in the same cache. To disable > - merging at runtime, "slab_nomerge" can be passed on the kernel > + can usually only damage objects in the same cache. To enable > + merging at runtime, "slab_merge" can be passed on the kernel > command line. > > config SLAB_FREELIST_RANDOM > -- > 2.40.1 > > --2003067076-1055337799-1687894336=:2486373--