Re: [PATCH v9 1/8] mm: Introduce memfd_restricted system call to create restricted user memory

[Chao Peng]

On Mon, Nov 28, 2022 at 06:37:25PM -0600, Michael Roth wrote:
> On Tue, Oct 25, 2022 at 11:13:37PM +0800, Chao Peng wrote:
...
> > +static long restrictedmem_fallocate(struct file *file, int mode,
> > +				    loff_t offset, loff_t len)
> > +{
> > +	struct restrictedmem_data *data = file->f_mapping->private_data;
> > +	struct file *memfd = data->memfd;
> > +	int ret;
> > +
> > +	if (mode & FALLOC_FL_PUNCH_HOLE) {
> > +		if (!PAGE_ALIGNED(offset) || !PAGE_ALIGNED(len))
> > +			return -EINVAL;
> > +	}
> > +
> > +	restrictedmem_notifier_invalidate(data, offset, offset + len, true);
> 
> The KVM restrictedmem ops seem to expect pgoff_t, but here we pass
> loff_t. For SNP we've made this strange as part of the following patch
> and it seems to produce the expected behavior:

That's correct. Thanks.

> 
>   https://github.com/mdroth/linux/commit/d669c7d3003ff7a7a47e73e8c3b4eeadbd2c4eb6
> 
> > +	ret = memfd->f_op->fallocate(memfd, mode, offset, len);
> > +	restrictedmem_notifier_invalidate(data, offset, offset + len, false);
> > +	return ret;
> > +}
> > +
> 
> <snip>
> 
> > +int restrictedmem_get_page(struct file *file, pgoff_t offset,
> > +			   struct page **pagep, int *order)
> > +{
> > +	struct restrictedmem_data *data = file->f_mapping->private_data;
> > +	struct file *memfd = data->memfd;
> > +	struct page *page;
> > +	int ret;
> > +
> > +	ret = shmem_getpage(file_inode(memfd), offset, &page, SGP_WRITE);
> 
> This will result in KVM allocating pages that userspace hasn't necessary
> fallocate()'d. In the case of SNP we need to get the PFN so we can clean
> up the RMP entries when restrictedmem invalidations are issued for a GFN
> range.

Yes fallocate() is unnecessary unless someone wants to reserve some
space (e.g. for determination or performance purpose), this matches its
semantics perfectly at:
https://www.man7.org/linux/man-pages/man2/fallocate.2.html

> 
> If the guest supports lazy-acceptance however, these pages may not have
> been faulted in yet, and if the VMM defers actually fallocate()'ing space
> until the guest actually tries to issue a shared->private for that GFN
> (to support lazy-pinning), then there may never be a need to allocate
> pages for these backends.
> 
> However, the restrictedmem invalidations are for GFN ranges so there's
> no way to know inadvance whether it's been allocated yet or not. The
> xarray is one option but currently it defaults to 'private' so that
> doesn't help us here. It might if we introduced a 'uninitialized' state
> or something along that line instead of just the binary
> 'shared'/'private' though...

How about if we change the default to 'shared' as we discussed at
https://lore.kernel.org/all/Y35gI0L8GMt9+OkK@xxxxxxxxxx/?
> 
> But for now we added a restrictedmem_get_page_noalloc() that uses
> SGP_NONE instead of SGP_WRITE to avoid accidentally allocating a bunch
> of memory as part of guest shutdown, and a
> kvm_restrictedmem_get_pfn_noalloc() variant to go along with that. But
> maybe a boolean param is better? Or maybe SGP_NOALLOC is the better
> default, and we just propagate an error to userspace if they didn't
> fallocate() in advance?

This (making fallocate() a hard requirement) not only complicates the
userspace but also forces the lazy-faulting going through a long path of
exiting to userspace. Unless we don't have other options I would not go
this way.

Chao
> 
> -Mike
> 
> > +	if (ret)
> > +		return ret;
> > +
> > +	*pagep = page;
> > +	if (order)
> > +		*order = thp_order(compound_head(page));
> > +
> > +	SetPageUptodate(page);
> > +	unlock_page(page);
> > +
> > +	return 0;
> > +}
> > +EXPORT_SYMBOL_GPL(restrictedmem_get_page);
> > -- 
> > 2.25.1
> >