Intruduce stacking filesystem vfs helpers

Christian,

These patches essentially just lift some overlayfs code to common
file fs/backing_file.c.

They are based on vfs.rw and overlayfs-next branches.

The motivation is (per Miklos' request) to reuse common stacking code
for the FUSE passthrough patches that I am shaping up for upstream.

I have been testing those patches with my fuse-backing-fd development
branch [1] for quite some time and I think both you and Miklos gave
a conceptual ACK to some version of this work, but the patches were
never publicly posted.

I am aiming the FUSE passthough work for the 6.9 dev cycle and I was
contemplating whether I should queue these up for 6.8 merge window to
give them time to mature before 6.9 merge window.

If I do that, would you preffer to take these patches via the vfs tree
or will you be ok with me including them in the overlayfs PR for 6.8?

Miklos,

Are you ok with including this in overlayfs PR for 6.8?

Thanks,
Amir.

[1] https://github.com/amir73il/linux/commits/fuse-backing-fd

Amir Goldstein (4):
  fs: prepare for stackable filesystems backing file helpers
  fs: factor out backing_file_{read,write}_iter() helpers
  fs: factor out backing_file_splice_{read,write}() helpers
  fs: factor out backing_file_mmap() helper

 MAINTAINERS                  |   9 +
 fs/Kconfig                   |   4 +
 fs/Makefile                  |   1 +
 fs/backing-file.c            | 324 +++++++++++++++++++++++++++++++++++
 fs/open.c                    |  38 ----
 fs/overlayfs/Kconfig         |   1 +
 fs/overlayfs/file.c          | 245 ++++----------------------
 fs/overlayfs/overlayfs.h     |   8 +-
 fs/overlayfs/super.c         |  11 +-
 include/linux/backing-file.h |  42 +++++
 include/linux/fs.h           |   3 -
 11 files changed, 423 insertions(+), 263 deletions(-)
 create mode 100644 fs/backing-file.c
 create mode 100644 include/linux/backing-file.h

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2.34.1