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

Michael Roth
2022-11-29T19:07:26Z

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&amp;data=05%7C01%7Cmichael.roth%40amd.com%7C99e80696067a40d42f6e08dad2138556%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C638053278531323330%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&amp;sdata=WDj4KxJjhcntBWJUGCjNmMPfZMGQkCSaAo6ElYrGgF0%3D&amp;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%7C99e80696067a40d42f6e08dad2138556%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C638053278531323330%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&amp;sdata=67sdTY47cM1IBUG2eJCltYF5SyGOpd9%2FVxVlHUw02tU%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%7C99e80696067a40d42f6e08dad2138556%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C638053278531323330%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&amp;sdata=qzWObDo7ZHW4YjuAjZ5%2B1wEwbqymgBiNM%2BYXiyUSBdI%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.

-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
> > >