linux-kernel - Re: [PATCH v6 5/6] mm: secretmem: use PMD-size pages to amortize direct map fragmentation

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Date:   Tue, 29 Sep 2020 07:31:08 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Mike Rapoport <rppt@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christopher Lameter <cl@...ux.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>, Idan Yaniv <idan.yaniv@....com>,
        Ingo Molnar <mingo@...hat.com>,
        James Bottomley <jejb@...ux.ibm.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Matthew Wilcox <willy@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Shuah Khan <shuah@...nel.org>, Tycho Andersen <tycho@...ho.ws>,
        Will Deacon <will@...nel.org>, linux-api@...r.kernel.org,
        linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-riscv@...ts.infradead.org,
        x86@...nel.org
Subject: Re: [PATCH v6 5/6] mm: secretmem: use PMD-size pages to amortize
 direct map fragmentation

On 9/29/20 7:12 AM, Peter Zijlstra wrote:
>>                              |  1G    |  2M    |  4K
>>        ----------------------+--------+--------+---------
>>   ssd, mitigations=on	| 308.75 | 317.37 | 314.9
>>   ssd, mitigations=off	| 305.25 | 295.32 | 304.92
>>   ram, mitigations=on	| 301.58 | 322.49 | 306.54
>>   ram, mitigations=off	| 299.32 | 288.44 | 310.65
> These results lack error data, but assuming the reults are significant,
> then this very much makes a case for 1G mappings. 5s on a kernel builds
> is pretty good.

Is something like secretmem all or nothing?

This seems like a similar situation to the side-channel mitigations.  We
know what the most "secure" thing to do is.  But, folks also disagree
about how much pain that security is worth.

That seems to indicate we're never going to come up with a
one-size-fits-all solution to this.  Apps are going to have to live
without secretmem being around if they want to run on old kernels
anyway, so it seems like something we should be able to enable or
disable without ABI concerns.

Do we just include it, but disable it by default so it doesn't eat
performance?  But, allow it to be reenabled by the folks who generally
prioritize hardening over performance, like Chromebooks for instance.