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Subject: Re: [PATCH v16 07/11] secretmem: use PMD-size pages to amortize
 direct map fragmentation
To:     Michal Hocko <mhocko@suse.com>
Cc:     Mike Rapoport <rppt@kernel.org>,
        James Bottomley <jejb@linux.ibm.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andy Lutomirski <luto@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>, Borislav Petkov <bp@alien8.de>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Christopher Lameter <cl@linux.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Elena Reshetova <elena.reshetova@intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        Matthew Wilcox <willy@infradead.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Rick Edgecombe <rick.p.edgecombe@intel.com>,
        Roman Gushchin <guro@fb.com>,
        Shakeel Butt <shakeelb@google.com>,
        Shuah Khan <shuah@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Tycho Andersen <tycho@tycho.ws>, Will Deacon <will@kernel.org>,
        linux-api@vger.kernel.org, linux-arch@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
        linux-nvdimm@lists.01.org, linux-riscv@lists.infradead.org,
        x86@kernel.org, Hagen Paul Pfeifer <hagen@jauu.net>,
        Palmer Dabbelt <palmerdabbelt@google.com>
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 <YBlgCl8MQuuII22w@dhcp22.suse.cz>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <d4fe580a-ef0e-e13f-9ee4-16fb8b6d65dd@redhat.com>
Date:   Tue, 2 Feb 2021 15:26:20 +0100
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On 02.02.21 15:22, Michal Hocko wrote:
> On Tue 02-02-21 15:12:21, David Hildenbrand wrote:
> [...]
>> I think secretmem behaves much more like longterm GUP right now
>> ("unmigratable", "lifetime controlled by user space", "cannot go on
>> CMA/ZONE_MOVABLE"). I'd either want to reasonably well control/limit it or
>> make it behave more like mlocked pages.
> 
> I thought I have already asked but I must have forgotten. Is there any
> actual reason why the memory is not movable? Timing attacks?

I think the reason is simple: no direct map, no copying of memory.

As I mentioned, we would have to temporarily map in order to copy. 
Mapping it somewhere else (like kmap), outside of the direct map might 
reduce possible attacks.

-- 
Thanks,

David / dhildenb