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Subject: Re: [PATCH v6 0/6] mm: introduce memfd_secret system call to create
 "secret" memory areas
To:     Mike Rapoport <rppt@kernel.org>
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>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Shuah Khan <shuah@kernel.org>, 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,
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        linux-nvdimm@lists.01.org, linux-riscv@lists.infradead.org,
        x86@kernel.org
References: <20200924132904.1391-1-rppt@kernel.org>
 <9c38ac3b-c677-6a87-ce82-ec53b69eaf71@redhat.com>
 <20201102174308.GF4879@kernel.org>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat GmbH
Message-ID: <d4cb2c87-4744-3929-cedd-2be78625a741@redhat.com>
Date:   Mon, 2 Nov 2020 18:51:09 +0100
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>> 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.

> 
> But again, there is nothing in the current form of secretmem that
> prevents allocation from ZONE_MOVABLE.

Oh, there is something: That the pages are not movable.

-- 
Thanks,

David / dhildenb