Re: [PATCH v10 1/9] mm: Introduce memfd_restricted system call to create restricted user memory

[Huang, Kai]

On Mon, 2023-01-23 at 15:03 +0100, Vlastimil Babka wrote:
> On 12/22/22 01:37, Huang, Kai wrote:
> > > > I argue that this page pinning (or page migration prevention) is not
> > > > tied to where the page comes from, instead related to how the page will
> > > > be used. Whether the page is restrictedmem backed or GUP() backed, once
> > > > it's used by current version of TDX then the page pinning is needed. So
> > > > such page migration prevention is really TDX thing, even not KVM generic
> > > > thing (that's why I think we don't need change the existing logic of
> > > > kvm_release_pfn_clean()). 
> > > > 
> > This essentially boils down to who "owns" page migration handling, and sadly,
> > page migration is kinda "owned" by the core-kernel, i.e. KVM cannot handle page
> > migration by itself -- it's just a passive receiver.
> > 
> > For normal pages, page migration is totally done by the core-kernel (i.e. it
> > unmaps page from VMA, allocates a new page, and uses migrate_pape() or a_ops-
> > > migrate_page() to actually migrate the page).
> > In the sense of TDX, conceptually it should be done in the same way. The more
> > important thing is: yes KVM can use get_page() to prevent page migration, but
> > when KVM wants to support it, KVM cannot just remove get_page(), as the core-
> > kernel will still just do migrate_page() which won't work for TDX (given
> > restricted_memfd doesn't have a_ops->migrate_page() implemented).
> > 
> > So I think the restricted_memfd filesystem should own page migration handling,
> > (i.e. by implementing a_ops->migrate_page() to either just reject page migration
> > or somehow support it).
> 
> While this thread seems to be settled on refcounts already, 
> 

I am not sure but will let Sean/Paolo to decide.

> just wanted
> to point out that it wouldn't be ideal to prevent migrations by
> a_ops->migrate_page() rejecting them. It would mean cputime wasted (i.e.
> by memory compaction) by isolating the pages for migration and then
> releasing them after the callback rejects it (at least we wouldn't waste
> time creating and undoing migration entries in the userspace page tables
> as there's no mmap). Elevated refcount on the other hand is detected
> very early in compaction so no isolation is attempted, so from that
> aspect it's optimal.

I am probably missing something, but IIUC the checking of refcount happens at
very last stage of page migration too, for instance:

	migrate_folio(...) ->
		migrate_folio_extra(..., 0 /* extra_count */) ->
			folio_migrate_mapping(...).

And it is folio_migrate_mapping() who does the actual compare with the refcount,
which is at very late stage too:

int folio_migrate_mapping(struct address_space *mapping,
                struct folio *newfolio, struct folio *folio, int extra_count)
{
	...
        int expected_count = folio_expected_refs(mapping, folio) + extra_count;

        if (!mapping) {
                /* Anonymous page without mapping */
                if (folio_ref_count(folio) != expected_count)
                        return -EAGAIN;

		....
                return MIGRATEPAGE_SUCCESS;
        }

	....
        if (!folio_ref_freeze(folio, expected_count)) {
                xas_unlock_irq(&xas);
                return -EAGAIN;
        }
	...
}