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

[Chao Peng]

On Thu, Dec 22, 2022 at 12:37:19AM +0000, Huang, Kai wrote:
> On Wed, 2022-12-21 at 21:39 +0800, 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:
> > > > > > > > > > > > > 
> > > > > > > > > > > > > [...]
> > > > > > > > > > > > > 
> > > > > > > > > > > > > > > +
> > > > > > > > > > > > > > > + /*
> > > > > > > > > > > > > > > +  * These pages are currently unmovable so don't 
> > > > > > > > > > > > > > > place them into
> > > > > > > > > > > > > > > movable
> > > > > > > > > > > > > > > +  * pageblocks (e.g. CMA and ZONE_MOVABLE).
> > > > > > > > > > > > > > > +  */
> > > > > > > > > > > > > > > + mapping = memfd->f_mapping;
> > > > > > > > > > > > > > > + mapping_set_unevictable(mapping);
> > > > > > > > > > > > > > > + mapping_set_gfp_mask(mapping,
> > > > > > > > > > > > > > > +                      mapping_gfp_mask(mapping) 
> > > > > > > > > > > > > > > & ~__GFP_MOVABLE);
> > > > > > > > > > > > > 
> > > > > > > > > > > > > But, IIUC removing __GFP_MOVABLE flag here only makes 
> > > > > > > > > > > > > page allocation from
> > > > > > > > > > > > > non-
> > > > > > > > > > > > > movable zones, but doesn't necessarily prevent page 
> > > > > > > > > > > > > from being migrated.  My
> > > > > > > > > > > > > first glance is you need to implement either 
> > > > > > > > > > > > > a_ops->migrate_folio() or just
> > > > > > > > > > > > > get_page() after faulting in the page to prevent.
> > > > > > > > > > > 
> > > > > > > > > > > The current api restrictedmem_get_page() already does 
> > > > > > > > > > > this, after the
> > > > > > > > > > > caller calling it, it holds a reference to the page. The 
> > > > > > > > > > > caller then
> > > > > > > > > > > decides when to call put_page() appropriately.
> > > > > > > > > 
> > > > > > > > > I tried to dig some history. Perhaps I am missing something, 
> > > > > > > > > but it seems Kirill
> > > > > > > > > said in v9 that this code doesn't prevent page migration, and 
> > > > > > > > > we need to
> > > > > > > > > increase page refcount in restrictedmem_get_page():
> > > > > > > > > 
> > > > > > > > > https://lore.kernel.org/linux-mm/20221129112139.usp6dqhbih47qpjl@box.shutemov.name/
> > > > > > > > > 
> > > > > > > > > But looking at this series it seems restrictedmem_get_page() 
> > > > > > > > > in this v10 is
> > > > > > > > > identical to the one in v9 (except v10 uses 'folio' instead 
> > > > > > > > > of 'page')?
> > > > > > > 
> > > > > > > restrictedmem_get_page() increases page refcount several versions 
> > > > > > > ago so
> > > > > > > no change in v10 is needed. You probably missed my reply:
> > > > > > > 
> > > > > > > https://lore.kernel.org/linux-mm/20221129135844.GA902164@chaop.bj.intel.com/
> > > > > 
> > > > > 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.
> 
> OK. Agreed.
> 
> > > 
> > > > > 
> > > > > 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.
> > > 
> > > 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.

No, I'm not talking on the page migration handling itself, I know page
migration requires coordination from both core-mm and KVM. I'm more
concerning on the page migration prevention here. This is something we
need to address for TDX before the page migration is supported.

> 
> 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).
> 
> To support page migration, it may require KVM's help in case of TDX (the
> TDH.MEM.PAGE.RELOCATE SEAMCALL requires "GPA" and "level" of EPT mapping, 
> which
> are only available in KVM), but that doesn't make KVM to own the handling of
> page migration.
> 
> 
> > > Wouldn't better to let TDX code (or who
> > > requires that) to increase/decrease the refcount when it populates/drops
> > > the secure EPT entries? This is exactly what the current TDX code does:
> > > 
> > > get_page():
> > > https://github.com/intel/tdx/blob/kvm-upstream/arch/x86/kvm/vmx/tdx.c#L1217
> > > 
> > > put_page():
> > > https://github.com/intel/tdx/blob/kvm-upstream/arch/x86/kvm/vmx/tdx.c#L1334
> > > 
> 
> As explained above, I think doing so in KVM is wrong: it can prevent by using
> get_page(), but you cannot simply remove it to support page migration.

Removing get_page() is definitely not enough for page migration support.
But the key thing is for page migration prevention, other than
get_page(), do we really have alternative.

Thanks,
Chao
> 
> Sean also said similar thing when reviewing v8 KVM TDX series and I also 
> agree:
> 
> https://lore.kernel.org/lkml/Yvu5PsAndEbWKTHc@google.com/
> https://lore.kernel.org/lkml/31fec1b4438a6d9bb7ff719f96caa8b23ed764d6.camel@intel.com/
>