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

Mike Rapoport
2021-02-11T07:14:00Z

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.