Re: [PATCH v5 04/13] mm/shmem: Restrict MFD_INACCESSIBLE memory against RLIMIT_MEMLOCK

[Hugh Dickins]

On Mon, 11 Apr 2022, Kirill A. Shutemov wrote:
> On Fri, Apr 08, 2022 at 09:02:54PM +0800, Chao Peng wrote:
> > > I think the correct approach is to not do the locking automatically for SHM_F_INACCESSIBLE,
> > > and instead require userspace to do shmctl(.., SHM_LOCK, ...) if userspace knows the
> > > consumers don't support migrate/swap.  That'd require wrapping migrate_page() and then
> > > wiring up notifier hooks for migrate/swap, but IMO that's a good thing to get sorted
> > > out sooner than later.  KVM isn't planning on support migrate/swap for TDX or SNP,
> > > but supporting at least migrate for a software-only implementation a la pKVM should
> > > be relatively straightforward.  On the notifiee side, KVM can terminate the VM if it
> > > gets an unexpected migrate/swap, e.g. so that TDX/SEV VMs don't die later with
> > > exceptions and/or data corruption (pre-SNP SEV guests) in the guest.
> > 
> > SHM_LOCK sounds like a good match.
> 
> Emm, no. shmctl(2) and SHM_LOCK are SysV IPC thing. I don't see how they
> fit here.

I am still struggling to formulate a constructive response on
MFD_INACCESSIBLE in general: but before doing so, let me jump in here
to say that I'm firmly on the side of SHM_LOCK being the right model -
but admittedly not through userspace calling shmctl(2).

Please refer to our last year's posting "[PATCH 10/16] tmpfs: fcntl(fd,
F_MEM_LOCK) to memlock a tmpfs file" for the example of how Shakeel did
it then (though only a small part of that would be needed for this case):
https://lore.kernel.org/linux-mm/54e03798-d836-ae64-f41-4a1d46bc115b@xxxxxxxxxx/

And until such time as swapping is enabled, this memlock accounting would
be necessarily entailed by "MFD_INACCESSIBLE", or however that turns out
to be implemented: not something that we could trust userspace to call
separately.

Hugh