Hi, Currently, guest_memfd doesn't update inode's i_blocks or i_bytes at all. Hence, st_blocks in the struct populated by a userspace fstat() call on a guest_memfd will always be 0. This patch series makes guest_memfd track the amount of memory allocated on an inode, which allows fstat() to accurately report that on requests from userspace.
The inode's i_blocks and i_bytes fields are updated when the folio is associated or disassociated from the guest_memfd inode, which are at allocation and truncation times respectively. In RFC v2, to update inode fields at truncation time, this series adds a new .unaccount_folio callback to struct address_space_operations, which guest_memfd uses to deduct the folio's size at truncation time. The second patch, to use filemap_alloc_folio() during allocation of guest_memfd folios, was written as a debugging step to resolve a bug found by syzbot [1], but turned out to not be the fix. I include it here because it cleans up the allocation process and provides a nice foundation for updating inode fields during allocations. The first patch was separately submitted [2], and provided here since it is a prerequisite simplication before application of the second patch. Changes from RFC v1: + Removed a full custom implementation of .evict_inode for guest_memfd in favor of adding .unaccount_folio callback. RFC v1: https://lore.kernel.org/all/[email protected]/T/ [1] https://lore.kernel.org/all/29c347bde68ec027259654e8e85371307edf7058.1770148108.git.ackerley...@google.com/ [2] https://lore.kernel.org/all/[email protected]/ Signed-off-by: Ackerley Tng <[email protected]> --- Ackerley Tng (6): KVM: guest_memfd: Don't set FGP_ACCESSED when getting folios KVM: guest_memfd: Directly allocate folios with filemap_alloc_folio() fs: Add .unaccount_folio callback KVM: guest_memfd: Track amount of memory allocated on inode KVM: selftests: Wrap fstat() to assert success KVM: selftests: Test that st_blocks is updated on allocation Documentation/filesystems/vfs.rst | 8 +++ include/linux/fs.h | 1 + mm/filemap.c | 3 ++ tools/testing/selftests/kvm/guest_memfd_test.c | 32 ++++++++---- tools/testing/selftests/kvm/include/kvm_syscalls.h | 2 + virt/kvm/guest_memfd.c | 60 ++++++++++++++++------ 6 files changed, 80 insertions(+), 26 deletions(-) --- base-commit: b1195183ed42f1522fae3fe44ebee3af437aa000 change-id: 20260225-gmem-st-blocks-733f35d10211 Best regards, -- Ackerley Tng <[email protected]>
