Date:   Mon, 23 Jan 2023 23:38:07 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Huang, Kai" <kai.huang@...el.com>
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Subject: Re: [PATCH v10 1/9] mm: Introduce memfd_restricted system call to
 create restricted user memory

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

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

> > 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 

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