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Date:   Tue, 2 Feb 2021 15:32:16 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     David Hildenbrand <david@redhat.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>,
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        Palmer Dabbelt <palmer@dabbelt.com>,
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        Shakeel Butt <shakeelb@google.com>,
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Subject: Re: [PATCH v16 07/11] secretmem: use PMD-size pages to amortize
 direct map fragmentation
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On Tue 02-02-21 15:26:20, David Hildenbrand wrote:
> 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.

This is an implementation detail though and not something terribly hard
to add on top later on. I was more worried there would be really
fundamental reason why this is not possible. E.g. security implications.
-- 
Michal Hocko
SUSE Labs