On Mon, Jan 23, 2023, Huang, Kai wrote:

On Mon, 2023-01-23 at 15:03 +0100, Vlastimil Babka wrote:

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On 12/22/22 01:37, Huang, Kai wrote:

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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-

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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.

My preference is whatever is most performant without being hideous :-)

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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,

Heh, me too, I could have sworn that using refcounts was the least efficient way
to block migration.

but IIUC the checking of refcount happens at very last stage of page migration too