Re: [PATCH 3/5] KVM: gmem: Hold filemap invalidate lock while allocating/preparing folios

[Vishal Annapurve]

On Mon, Jun 2, 2025 at 6:34 PM Yan Zhao <yan.y.zhao@xxxxxxxxx> wrote:
>
> On Mon, Jun 02, 2025 at 06:05:32PM -0700, Vishal Annapurve wrote:
> > On Tue, May 20, 2025 at 11:49 PM Yan Zhao <yan.y.zhao@xxxxxxxxx> wrote:
> > >
> > > On Mon, May 19, 2025 at 10:04:45AM -0700, Ackerley Tng wrote:
> > > > Ackerley Tng <ackerleytng@xxxxxxxxxx> writes:
> > > >
> > > > > Yan Zhao <yan.y.zhao@xxxxxxxxx> writes:
> > > > >
> > > > >> On Fri, Mar 14, 2025 at 05:20:21PM +0800, Yan Zhao wrote:
> > > > >>> This patch would cause host deadlock when booting up a TDX VM even if huge page
> > > > >>> is turned off. I currently reverted this patch. No further debug yet.
> > > > >> This is because kvm_gmem_populate() takes filemap invalidation lock, and for
> > > > >> TDX, kvm_gmem_populate() further invokes kvm_gmem_get_pfn(), causing deadlock.
> > > > >>
> > > > >> kvm_gmem_populate
> > > > >>   filemap_invalidate_lock
> > > > >>   post_populate
> > > > >>     tdx_gmem_post_populate
> > > > >>       kvm_tdp_map_page
> > > > >>        kvm_mmu_do_page_fault
> > > > >>          kvm_tdp_page_fault
> > > > >>       kvm_tdp_mmu_page_fault
> > > > >>         kvm_mmu_faultin_pfn
> > > > >>           __kvm_mmu_faultin_pfn
> > > > >>             kvm_mmu_faultin_pfn_private
> > > > >>               kvm_gmem_get_pfn
> > > > >>                 filemap_invalidate_lock_shared
> > > > >>
> > > > >> Though, kvm_gmem_populate() is able to take shared filemap invalidation lock,
> > > > >> (then no deadlock), lockdep would still warn "Possible unsafe locking scenario:
> > > > >> ...DEADLOCK" due to the recursive shared lock, since commit e918188611f0
> > > > >> ("locking: More accurate annotations for read_lock()").
> > > > >>
> > > > >
> > > > > Thank you for investigating. This should be fixed in the next revision.
> > > > >
> > > >
> > > > This was not fixed in v2 [1], I misunderstood this locking issue.
> > > >
> > > > IIUC kvm_gmem_populate() gets a pfn via __kvm_gmem_get_pfn(), then calls
> > > > part of the KVM fault handler to map the pfn into secure EPTs, then
> > > > calls the TDX module for the copy+encrypt.
> > > >
> > > > Regarding this lock, seems like KVM'S MMU lock is already held while TDX
> > > > does the copy+encrypt. Why must the filemap_invalidate_lock() also be
> > > > held throughout the process?
> > > If kvm_gmem_populate() does not hold filemap invalidate lock around all
> > > requested pages, what value should it return after kvm_gmem_punch_hole() zaps a
> > > mapping it just successfully installed?
> > >
> > > TDX currently only holds the read kvm->mmu_lock in tdx_gmem_post_populate() when
> > > CONFIG_KVM_PROVE_MMU is enabled, due to both slots_lock and the filemap
> > > invalidate lock being taken in kvm_gmem_populate().
> >
> > Does TDX need kvm_gmem_populate path just to ensure SEPT ranges are
> > not zapped during tdh_mem_page_add and tdh_mr_extend operations? Would
> > holding KVM MMU read lock during these operations sufficient to avoid
> > having to do this back and forth between TDX and gmem layers?
> I think the problem here is because in kvm_gmem_populate(),
> "__kvm_gmem_get_pfn(), post_populate(), and kvm_gmem_mark_prepared()"
> must be wrapped in filemap invalidate lock (shared or exclusive), right?
>
> Then, in TDX's post_populate() callback, the filemap invalidate lock is held
> again by kvm_tdp_map_page() --> ... ->kvm_gmem_get_pfn().

I am contesting the need of kvm_gmem_populate path altogether for TDX.
Can you help me understand what problem does kvm_gmem_populate path
help with for TDX?