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 4cd071d13c ("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 8dd861cc07 ("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: 2f6305dd56 ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Nikolay Kuratov
0d5c7fcfa5