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

[Sean Christopherson]

On Thu, Mar 10, 2022, Chao Peng wrote:
> Since page migration / swapping is not supported yet, MFD_INACCESSIBLE
> memory behave like longterm pinned pages and thus should be accounted to
> mm->pinned_vm and be restricted by RLIMIT_MEMLOCK.
> 
> Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> ---
>  mm/shmem.c | 25 ++++++++++++++++++++++++-
>  1 file changed, 24 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 7b43e274c9a2..ae46fb96494b 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -915,14 +915,17 @@ static void notify_fallocate(struct inode *inode, pgoff_t start, pgoff_t end)
>  static void notify_invalidate_page(struct inode *inode, struct folio *folio,
>  				   pgoff_t start, pgoff_t end)
>  {
> -#ifdef CONFIG_MEMFILE_NOTIFIER
>  	struct shmem_inode_info *info = SHMEM_I(inode);
>  
> +#ifdef CONFIG_MEMFILE_NOTIFIER
>  	start = max(start, folio->index);
>  	end = min(end, folio->index + folio_nr_pages(folio));
>  
>  	memfile_notifier_invalidate(&info->memfile_notifiers, start, end);
>  #endif
> +
> +	if (info->xflags & SHM_F_INACCESSIBLE)
> +		atomic64_sub(end - start, &current->mm->pinned_vm);

As Vishal's to-be-posted selftest discovered, this is broken as current->mm may
be NULL.  Or it may be a completely different mm, e.g. AFAICT there's nothing that
prevents a different process from punching hole in the shmem backing.

I don't see a sane way of tracking this in the backing store unless the inode is
associated with a single mm when it's created, and that opens up a giant can of
worms, e.g. what happens with the accounting if the creating process goes away?

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.

Hmm, shmem_writepage() already handles SHM_F_INACCESSIBLE by rejecting the swap, so
maybe it's just the page migration path that needs to be updated?