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

[Michael Roth]

On Tue, Nov 29, 2022 at 01:06:58PM -0600, Michael Roth wrote:
> On Tue, Nov 29, 2022 at 10:06:15PM +0800, Chao Peng wrote:
> > 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://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fmdroth%2Flinux%2Fcommit%2Fd669c7d3003ff7a7a47e73e8c3b4eeadbd2c4eb6&data=05%7C01%7CMichael.Roth%40amd.com%7C0c26815eb6af4f1a243508dad23cf713%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C638053456609134623%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=kAL42bmyBB0alVwh%2FN%2BT3D%2BiVTdxxMsJ7V4TNuCTjM4%3D&reserved=0
> > > 
> > > > +	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://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.man7.org%2Flinux%2Fman-pages%2Fman2%2Ffallocate.2.html&amp;data=05%7C01%7CMichael.Roth%40amd.com%7C0c26815eb6af4f1a243508dad23cf713%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C638053456609134623%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&amp;sdata=acBSquFG%2FHtpbcZfHDZrP2O63bu06rI0pjiPJFSJSj8%3D&amp;reserved=0
> > 
> > > 
> > > 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://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fall%2FY35gI0L8GMt9%2BOkK%40google.com%2F&amp;data=05%7C01%7CMichael.Roth%40amd.com%7C0c26815eb6af4f1a243508dad23cf713%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C638053456609134623%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&amp;sdata=Q1vZWQiZ7mx12Qn5aKl4s8Ea9hNbwCJBb%2BjiA1du3Os%3D&amp;reserved=0?
> 
> Need to look at this a bit more, but I think that could work as well.
> 
> > > 
> > > 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.
> 
> Unless I'm missing something, it's already the case that userspace is
> responsible for handling all the shared->private transitions in response
> to KVM_EXIT_MEMORY_FAULT or (in our case) KVM_EXIT_VMGEXIT. So it only
> places the additional requirements on the VMM that if they *don't*
> preallocate, then they'll need to issue the fallocate() prior to issuing
> the KVM_MEM_ENCRYPT_REG_REGION ioctl in response to these events.
> 
> QEMU for example already has a separate 'prealloc' option for cases
> where they want to prefault all the guest memory, so it makes sense to
> continue making that an optional thing with regard to UPM.

Although I guess what you're suggesting doesn't stop userspace from
deciding whether they want to prefault or not. I know the Google folks
had some concerns over unexpected allocations causing 2x memory usage
though so giving userspace full control of what is/isn't allocated in
the restrictedmem backend seems to make it easier to guard against this,
but I think checking the xarray and defaulting to 'shared' would work
for us if that's the direction we end up going.

-Mike

> 
> -Mike
> 
> > 
> > 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
> > > >