linux-kernel - Re: [PATCH v17 07/10] mm: introduce memfd_secret system call to create "secret" memory areas

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Message-ID: <20210211071319.GF242749@kernel.org>
Date:   Thu, 11 Feb 2021 09:13:19 +0200
From:   Mike Rapoport <rppt@...nel.org>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Mike Rapoport <rppt@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Catalin Marinas <catalin.marinas@....com>,
        Christopher Lameter <cl@...ux.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        David Hildenbrand <david@...hat.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        James Bottomley <jejb@...ux.ibm.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Matthew Wilcox <willy@...radead.org>,
        Mark Rutland <mark.rutland@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Roman Gushchin <guro@...com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Shuah Khan <shuah@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tycho Andersen <tycho@...ho.ws>, Will Deacon <will@...nel.org>,
        linux-api@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-riscv@...ts.infradead.org,
        x86@...nel.org, Hagen Paul Pfeifer <hagen@...u.net>,
        Palmer Dabbelt <palmerdabbelt@...gle.com>
Subject: Re: [PATCH v17 07/10] mm: introduce memfd_secret system call to
 create "secret" memory areas

On Tue, Feb 09, 2021 at 02:17:11PM +0100, Michal Hocko wrote:
> On Tue 09-02-21 11:09:38, Mike Rapoport wrote:
> > On Tue, Feb 09, 2021 at 09:47:08AM +0100, Michal Hocko wrote:
> > > 
> > > OK, so IIUC this means that the model is to hand over memory from host
> > > to guest. I thought the guest would be under control of its address
> > > space and therefore it operates on the VMAs. This would benefit from
> > > an additional and more specific clarification.
> > 
> > How guest would operate on VMAs if the interface between host and guest is
> > virtual hardware?
> 
> I have to say that I am not really familiar with this area so my view
> might be misleading or completely wrong. I thought that the HW address
> ranges are mapped to the guest process and therefore have a VMA.

There is a qemu process that currently has mappings of what guest sees as
its physical memory, but qemu is a part of hypervisor, i.e. host.
 
> > Citing my older email:
> > 
> >     I've hesitated whether to continue to use new flags to memfd_create() or to
> >     add a new system call and I've decided to use a new system call after I've
> >     started to look into man pages update. There would have been two completely
> >     independent descriptions and I think it would have been very confusing.
> 
> Could you elaborate? Unmapping from the kernel address space can work
> both for sealed or hugetlb memfds, no? Those features are completely
> orthogonal AFAICS. With a dedicated syscall you will need to introduce
> this functionality on top if that is required. Have you considered that?
> I mean hugetlb pages are used to back guest memory very often. Is this
> something that will be a secret memory usecase?
> 
> Please be really specific when giving arguments to back a new syscall
> decision.

Isn't "syscalls have completely independent description" specific enough?

We are talking about API here, not the implementation details whether
secretmem supports large pages or not.

The purpose of memfd_create() is to create a file-like access to memory.
The purpose of memfd_secret() is to create a way to access memory hidden
from the kernel.

I don't think overloading memfd_create() with the secretmem flags because
they happen to return a file descriptor will be better for users, but
rather will be more confusing.

-- 
Sincerely yours,
Mike.