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 kvm_page_fault field 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") Signed-off-by: Nikolay Kuratov kniv@yandex-team.ru --- arch/x86/kvm/mmu/mmu.c | 5 +++-- arch/x86/kvm/mmu/mmu_internal.h | 2 ++ arch/x86/kvm/mmu/paging_tmpl.h | 2 +- 3 files changed, 6 insertions(+), 3 deletions(-)
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c index 294775b7383b..2105f3bc2e59 100644 --- a/arch/x86/kvm/mmu/mmu.c +++ b/arch/x86/kvm/mmu/mmu.c @@ -4321,6 +4321,7 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault, smp_rmb();
ret = __kvm_faultin_pfn(vcpu, fault); + fault->faultin_pfn = fault->pfn; if (ret != RET_PF_CONTINUE) return ret;
@@ -4398,7 +4399,7 @@ 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(fault->faultin_pfn); return r; }
@@ -4474,7 +4475,7 @@ 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(fault->faultin_pfn); return r; } #endif diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h index decc1f153669..a016b51f9c62 100644 --- a/arch/x86/kvm/mmu/mmu_internal.h +++ b/arch/x86/kvm/mmu/mmu_internal.h @@ -236,6 +236,8 @@ struct kvm_page_fault { /* Outputs of kvm_faultin_pfn. */ unsigned long mmu_seq; kvm_pfn_t pfn; + /* pfn copy for kvm_release_pfn_clean(), constant after kvm_faultin_pfn() */ + kvm_pfn_t faultin_pfn; hva_t hva; bool map_writable;
diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h index c85255073f67..b945dde6e3be 100644 --- a/arch/x86/kvm/mmu/paging_tmpl.h +++ b/arch/x86/kvm/mmu/paging_tmpl.h @@ -848,7 +848,7 @@ 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(fault->faultin_pfn); return r; }