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

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Date:   Fri, 23 Dec 2022 00:50:22 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "Christopherson,, Sean" <seanjc@...gle.com>,
        "chao.p.peng@...ux.intel.com" <chao.p.peng@...ux.intel.com>
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        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
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Subject: Re: [PATCH v10 1/9] mm: Introduce memfd_restricted system call to
 create restricted user memory

On Thu, 2022-12-22 at 18:15 +0000, Sean Christopherson wrote:
> On Wed, Dec 21, 2022, Chao Peng wrote:
> > On Tue, Dec 20, 2022 at 08:33:05AM +0000, Huang, Kai wrote:
> > > On Tue, 2022-12-20 at 15:22 +0800, Chao Peng wrote:
> > > > On Mon, Dec 19, 2022 at 08:48:10AM +0000, Huang, Kai wrote:
> > > > > On Mon, 2022-12-19 at 15:53 +0800, Chao Peng wrote:
> > > But for non-restricted-mem case, it is correct for KVM to decrease page's
> > > refcount after setting up mapping in the secondary mmu, otherwise the page will
> > > be pinned by KVM for normal VM (since KVM uses GUP to get the page).
> > 
> > That's true. Actually even true for restrictedmem case, most likely we
> > will still need the kvm_release_pfn_clean() for KVM generic code. On one
> > side, other restrictedmem users like pKVM may not require page pinning
> > at all. On the other side, see below.
> > 
> > > 
> > > So what we are expecting is: for KVM if the page comes from restricted mem, then
> > > KVM cannot decrease the refcount, otherwise for normal page via GUP KVM should.
> 
> No, requiring the user (KVM) to guard against lack of support for page migration
> in restricted mem is a terrible API.  It's totally fine for restricted mem to not
> support page migration until there's a use case, but punting the problem to KVM
> is not acceptable.  Restricted mem itself doesn't yet support page migration,
> e.g. explosions would occur even if KVM wanted to allow migration since there is
> no notification to invalidate existing mappings.
> 
> 

Yes totally agree (I also replied separately).