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@google.com Signed-off-by: Sean Christopherson seanjc@google.com Reviewed-by: Sean Christopherson seanjc@google.com Signed-off-by: Nikolay Kuratov kniv@yandex-team.ru --- 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 | 10 ++++++++-- arch/x86/kvm/mmu/paging_tmpl.h | 5 ++++- 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index 294775b7383b..ff85526a9d48 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -4363,6 +4363,7 @@ static bool is_page_fault_stale(struct kvm_vcpu *vcpu,

static int direct_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault) { + kvm_pfn_t orig_pfn; int r;

/* Dummy roots are used only for shadowing bad guest roots. */ @@ -4384,6 +4385,8 @@ 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; write_lock(&vcpu->kvm->mmu_lock);

@@ -4398,7 +4401,7 @@ static int direct_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; }

@@ -4447,6 +4450,7 @@ EXPORT_SYMBOL_GPL(kvm_handle_page_fault); static int kvm_tdp_mmu_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault) { + kvm_pfn_t orig_pfn; int r;

if (page_fault_handle_page_track(vcpu, fault)) @@ -4464,6 +4468,8 @@ static int kvm_tdp_mmu_page_fault(struct kvm_vcpu *vcpu, if (r != RET_PF_CONTINUE) return r;

+ orig_pfn = fault->pfn; + r = RET_PF_RETRY; read_lock(&vcpu->kvm->mmu_lock);

@@ -4474,7 +4480,7 @@ static int kvm_tdp_mmu_page_fault(struct kvm_vcpu *vcpu,

out_unlock: read_unlock(&vcpu->kvm->mmu_lock); - kvm_release_pfn_clean(fault->pfn); + kvm_release_pfn_clean(orig_pfn); return r; } #endif diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h index c85255073f67..c6b2c52aceac 100644 --- a/arch/x86/kvm/mmu/paging_tmpl.h +++ b/arch/x86/kvm/mmu/paging_tmpl.h @@ -777,6 +777,7 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault, 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;

WARN_ON_ONCE(fault->is_tdp); @@ -835,6 +836,8 @@ 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);

@@ -848,7 +851,7 @@ 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; }