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Date:   Tue, 9 Feb 2021 21:08:31 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     Mike Rapoport <rppt@kernel.org>,
        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>,
        James Bottomley <jejb@linux.ibm.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>,
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        Shakeel Butt <shakeelb@google.com>,
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Subject: Re: [PATCH v17 00/10] mm: introduce memfd_secret system call to
 create "secret" memory areas
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On Tue 09-02-21 17:17:22, David Hildenbrand wrote:
> On 09.02.21 14:25, Michal Hocko wrote:
> > On Tue 09-02-21 11:23:35, David Hildenbrand wrote:
> > [...]
> > > I am constantly trying to fight for making more stuff MOVABLE instead of
> > > going into the other direction (e.g., because it's easier to implement,
> > > which feels like the wrong direction).
> > > 
> > > Maybe I am the only person that really cares about ZONE_MOVABLE these days
> > > :) I can't stop such new stuff from popping up, so at least I want it to be
> > > documented.
> > 
> > MOVABLE zone is certainly an important thing to keep working. And there
> > is still quite a lot of work on the way. But as I've said this is more
> > of a outlier than a norm. On the other hand movable zone is kinda hard
> > requirement for a lot of application and it is to be expected that
> > many features will be less than 100% compatible.  Some usecases even
> > impossible. That's why I am arguing that we should have a central
> > document where the movable zone is documented with all the potential
> > problems we have encountered over time and explicitly state which
> > features are fully/partially incompatible.
> > 
> 
> I'll send a mail during the next weeks to gather current restrictions to
> document them (and include my brain dump). We might see more excessive use
> of ZONE_MOVABLE in the future and as history told us, of CMA as well. We
> really should start documenting/caring.

Excellent! Thanks a lot. I will do my best to help reviewing that.

> @Mike, it would be sufficient for me if one of your patches at least mention
> the situation in the description like
> 
> "Please note that secretmem currently behaves much more like long-term GUP
> instead of mlocked memory; secretmem is unmovable memory directly
> consumed/controlled by user space. secretmem cannot be placed onto
> ZONE_MOVABLE/CMA.

Sounds good to me.

-- 
Michal Hocko
SUSE Labs