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

Vlastimil Babka
2023-02-13T14:23:52Z

On 1/23/23 16:18, Kirill A. Shutemov wrote:
> 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?

AS_UNMOVABLE should indeed allow a test in e.g. compaction to descide that
the page is not worth isolating in the first place.