[PATCH RFC v2 0/6] 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.

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/cover.1771826352.git.ackerleytng@xxxxxxxxxx/T/

[1] https://lore.kernel.org/all/29c347bde68ec027259654e8e85371307edf7058.1770148108.git.ackerleytng@xxxxxxxxxx/
[2] https://lore.kernel.org/all/20260129172646.2361462-1-ackerleytng@xxxxxxxxxx/

Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
---
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 <ackerleytng@xxxxxxxxxx>