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Date:   Tue, 3 Nov 2020 11:52:47 +0200
From:   Mike Rapoport <rppt@kernel.org>
To:     David Hildenbrand <david@redhat.com>
Cc:     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>, Idan Yaniv <idan.yaniv@ibm.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>,
        Paul Walmsley <paul.walmsley@sifive.com>,
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        Thomas Gleixner <tglx@linutronix.de>,
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        x86@kernel.org
Subject: Re: [PATCH v6 0/6] mm: introduce memfd_secret system call to create
 "secret" memory areas
Message-ID: <20201103095247.GH4879@kernel.org>
References: <20200924132904.1391-1-rppt@kernel.org>
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 <20201102174308.GF4879@kernel.org>
 <d4cb2c87-4744-3929-cedd-2be78625a741@redhat.com>
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On Mon, Nov 02, 2020 at 06:51:09PM +0100, David Hildenbrand wrote:
> > > Assume you have a system with quite some ZONE_MOVABLE memory (esp. in
> > > virtualized environments), eating up a significant amount of !ZONE_MOVABLE
> > > memory dynamically at runtime can lead to non-obvious issues. It looks like
> > > you have plenty of free memory, but the kernel might still OOM when trying
> > > to do kernel allocations e.g., for pagetables. With CMA we at least know
> > > what we're dealing with - it behaves like ZONE_MOVABLE except for the owner
> > > that can place unmovable pages there. We can use it to compute statically
> > > the amount of ZONE_MOVABLE memory we can have in the system without doing
> > > harm to the system.
> > 
> > Why would you say that secretmem allocates from !ZONE_MOVABLE?
> > If we put boot time reservations aside, the memory allocation for
> > secretmem follows the same rules as the memory allocations for any file
> > descriptor. That means we allocate memory with GFP_HIGHUSER_MOVABLE.
> 
> Oh, okay - I missed that! I had the impression that pages are unmovable and
> allocating from ZONE_MOVABLE would be a violation of that?
> 
> > After the allocation the memory indeed becomes unmovable but it's not
> > like we are eating memory from other zones here.
> 
> ... and here you have your problem. That's a no-no. We only allow it in very
> special cases where it can't be avoided - e.g., vfio having to pin guest
> memory when passing through memory to VMs.
> 
> Hotplug memory, online it to ZONE_MOVABLE. Allocate secretmem. Try to unplug
> the memory again -> endless loop in offline_pages().
> 
> Or have a CMA area that gets used with GFP_HIGHUSER_MOVABLE. Allocate
> secretmem. The owner of the area tries to allocate memory - always fails.
> Purpose of CMA destroyed.
> 
> > 
> > > Ideally, we would want to support page migration/compaction and allow for
> > > allocation from ZONE_MOVABLE as well. Would involve temporarily mapping,
> > > copying, unmapping. Sounds feasible, but not sure which roadblocks we would
> > > find on the way.
> > 
> > We can support migration/compaction with temporary mapping. The first
> > roadblock I've hit there was that migration allocates 4K destination
> > page and if we use it in secret map we are back to scrambling the direct
> > map into 4K pieces. It still sounds feasible but not as trivial :)
> 
> That sounds like the proper way for me to do it then.
 
Although migration of secretmem pages sounds feasible now, there maybe
other issues I didn't see because I'm not very familiar with
migration/compaction code.

I've looked again at CMA and I'm inclined to agree with you that using
CMA for secretmem allocations could be the right thing. 

-- 
Sincerely yours,
Mike.