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

Kirill A. Shutemov
2023-01-23T15:21:00Z

On Mon, Jan 23, 2023 at 03:03:45PM +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, 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.

Hm. Do we need a new hook in a_ops to check if the page is migratable
before going with longer path to migrate_page().

Or maybe add AS_UNMOVABLE?

--
Kiryl Shutsemau / Kirill A. Shutemov