[PATCH v2 6.1] KVM: x86/mmu: Ensure that kvm_release_pfn_clean() takes exact pfn from kvm_faultin_pfn()

[Nikolay Kuratov]

Since 5.16 and prior to 6.13 KVM can't be used with FSDAX
guest memory (PMD pages). To reproduce the issue you need to reserve
guest memory with `memmap=` cmdline, create and mount FS in DAX mode
(tested both XFS and ext4), see doc link below. ndctl command for test:
ndctl create-namespace -v -e namespace1.0 --map=dev --mode=fsdax -a 2M
Then pass memory object to qemu like:
-m 8G -object memory-backend-file,id=ram0,size=8G,\
mem-path=/mnt/pmem/guestmem,share=on,prealloc=on,dump=off,align=2097152 \
-numa node,memdev=ram0,cpus=0-1
QEMU fails to run guest with error: kvm run failed Bad address
and there are two warnings in dmesg:
WARN_ON_ONCE(!page_count(page)) in kvm_is_zone_device_page() and
WARN_ON_ONCE(folio_ref_count(folio) <= 0) in try_grab_folio() (v6.6.63)

It looks like in the past assumption was made that pfn won't change from
faultin_pfn() to release_pfn_clean(), e.g. see
commit 4cd071d13c5c ("KVM: x86/mmu: Move calls to thp_adjust() down a level")
But kvm_page_fault structure made pfn part of mutable state, so
now release_pfn_clean() can take hugepage-adjusted pfn.
And it works for all cases (/dev/shm, hugetlb, devdax) except fsdax.
Apparently in fsdax mode faultin-pfn and adjusted-pfn may refer to
different folios, so we're getting get_page/put_page imbalance.

To solve this preserve faultin pfn in separate local variable
and pass it in kvm_release_pfn_clean().

Patch tested for all mentioned guest memory backends with tdp_mmu={0,1}.

No bug in upstream as it was solved fundamentally by
commit 8dd861cc07e2 ("KVM: x86/mmu: Put refcounted pages instead of blindly releasing pfns")
and related patch series.

Link: https://nvdimm.docs.kernel.org/2mib_fs_dax.html
Fixes: 2f6305dd5676 ("KVM: MMU: change kvm_tdp_mmu_map() arguments to kvm_page_fault")
Co-developed-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Reviewed-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Nikolay Kuratov <kniv@xxxxxxxxxxxxxx>
---
v1 -> v2:
 * Instead of new struct field prefer local variable to snapshot faultin pfn
 as suggested by Sean Christopherson. 
 * Tested patch for 6.1 and 6.12

 arch/x86/kvm/mmu/mmu.c         | 5 ++++-
 arch/x86/kvm/mmu/paging_tmpl.h | 5 ++++-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 13134954e24d..d392022dcb89 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4245,6 +4245,7 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
 	bool is_tdp_mmu_fault = is_tdp_mmu(vcpu->arch.mmu);
 
 	unsigned long mmu_seq;
+	kvm_pfn_t orig_pfn;
 	int r;
 
 	fault->gfn = fault->addr >> PAGE_SHIFT;
@@ -4272,6 +4273,8 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
 	if (r != RET_PF_CONTINUE)
 		return r;
 
+	orig_pfn = fault->pfn;
+
 	r = RET_PF_RETRY;
 
 	if (is_tdp_mmu_fault)
@@ -4296,7 +4299,7 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
 		read_unlock(&vcpu->kvm->mmu_lock);
 	else
 		write_unlock(&vcpu->kvm->mmu_lock);
-	kvm_release_pfn_clean(fault->pfn);
+	kvm_release_pfn_clean(orig_pfn);
 	return r;
 }
 
diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index 1f4f5e703f13..685560a45bf6 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -790,6 +790,7 @@ FNAME(is_self_change_mapping)(struct kvm_vcpu *vcpu,
 static int FNAME(page_fault)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 {
 	struct guest_walker walker;
+	kvm_pfn_t orig_pfn;
 	int r;
 	unsigned long mmu_seq;
 	bool is_self_change_mapping;
@@ -868,6 +869,8 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
 			walker.pte_access &= ~ACC_EXEC_MASK;
 	}
 
+	orig_pfn = fault->pfn;
+
 	r = RET_PF_RETRY;
 	write_lock(&vcpu->kvm->mmu_lock);
 
@@ -881,7 +884,7 @@ static int FNAME(page_fault)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
 
 out_unlock:
 	write_unlock(&vcpu->kvm->mmu_lock);
-	kvm_release_pfn_clean(fault->pfn);
+	kvm_release_pfn_clean(orig_pfn);
 	return r;
 }
 
-- 
2.34.1