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. To update inode fields at truncation time, this series implements a custom truncation function for guest_memfd. An alternative would be to update truncate_inode_pages_range() to return the number of bytes truncated or add/use some hook. Implementing a custom truncation function was chosen to provide flexibility for handling truncations in future when guest_memfd supports sources of pages other than the buddy allocator. This approach of a custom truncation function also aligns with shmem, which has a custom shmem_truncate_range(). To update inode fields at allocation time, kvm_gmem_get_folio() is simply augmented such that when a folio is added to the filemap, the size of the folio is updated into inode fields. 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. [1] https://lore.kernel.org/all/29c347bde68ec027259654e8e85371307edf7058.1770148108.git.ackerley...@google.com/ [2] https://lore.kernel.org/all/[email protected]/ Ackerley Tng (10): KVM: guest_memfd: Don't set FGP_ACCESSED when getting folios KVM: guest_memfd: Directly allocate folios with filemap_alloc_folio() mm: truncate: Expose preparation steps for truncate_inode_pages_final() KVM: guest_memfd: Implement evict_inode for guest_memfd mm: Export unmap_mapping_folio() for KVM mm: filemap: Export filemap_remove_folio() KVM: guest_memfd: Implement custom truncation function 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 include/linux/mm.h | 3 + mm/filemap.c | 2 + mm/internal.h | 2 - mm/memory.c | 2 + mm/truncate.c | 21 +++- .../testing/selftests/kvm/guest_memfd_test.c | 32 +++-- .../selftests/kvm/include/kvm_syscalls.h | 2 + virt/kvm/guest_memfd.c | 116 +++++++++++++++--- 8 files changed, 149 insertions(+), 31 deletions(-) base-commit: b1195183ed42f1522fae3fe44ebee3af437aa000 -- 2.53.0.345.g96ddfc5eaa-goog
