[RFC PATCH v1 00/10] guest_memfd: Track amount of memory allocated on inode

[Ackerley Tng]

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.ackerleytng@xxxxxxxxxx/
[2] https://lore.kernel.org/all/20260129172646.2361462-1-ackerleytng@xxxxxxxxxx/

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