[PATCH RFC 00/20] file: remove f_version

[Christian Brauner <brauner@kernel.org>]

Probably not for this merge window anymore but let me get it out now:

The f_version member in struct file isn't particularly well-defined. It
is mainly used as a cookie to detect concurrent seeks when iterating
directories. But it is also abused by some subsystems for completely
unrelated things.

It is mostly a directory specific thing that doesn't really need to live
in struct file and with its wonky semantics it really lacks a specific
function.

For pipes, f_version is (ab)used to defer poll notifications until a
write has happened. And struct pipe_inode_info is used by multiple
struct files in their ->private_data so there's no chance of pushing
that down into file->private_data without introducing another pointer
indirection.

But this should be a solvable problem. Only regular files with
FMODE_ATOMIC_POS and directories require f_pos_lock. Pipes and other
files don't. So this adds a union into struct file encompassing
f_pos_lock and a pipe specific f_pipe member that pipes can use. This
union of course can be extended to other file types and is similar to
what we do in struct inode already.

I hope I got the details right. It at least holds up in xfstests and LTP
and my hand-rolled getdents-seek stressor.
  

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
Christian Brauner (20):
      file: remove pointless comment
      adi: remove unused f_version
      ceph: remove unused f_version
      s390: remove unused f_version
      fs: add vfs_setpos_cookie()
      fs: add must_set_pos()
      fs: use must_set_pos()
      fs: add generic_llseek_cookie()
      affs: store cookie in private data
      ext2: store cookie in private data
      ext4: store cookie in private data
      input: remove f_version abuse
      ocfs2: store cookie in private data
      proc: store cookie in private data
      udf: store cookie in private data
      ufs: store cookie in private data
      ubifs: store cookie in private data
      fs: add f_pipe
      pipe: use f_pipe
      fs: remove f_version

 drivers/char/adi.c             |   1 -
 drivers/input/input.c          |   8 +-
 drivers/s390/char/hmcdrv_dev.c |   3 -
 fs/affs/dir.c                  |  44 +++++++++--
 fs/ceph/dir.c                  |   1 -
 fs/ext2/dir.c                  |  28 ++++++-
 fs/ext4/dir.c                  |  50 ++++++------
 fs/ext4/ext4.h                 |   2 +
 fs/ext4/inline.c               |   7 +-
 fs/file_table.c                |   1 -
 fs/ocfs2/dir.c                 |   3 +-
 fs/ocfs2/file.c                |  11 ++-
 fs/ocfs2/file.h                |   1 +
 fs/pipe.c                      |   6 +-
 fs/proc/base.c                 |  18 +++--
 fs/read_write.c                | 169 +++++++++++++++++++++++++++++++----------
 fs/ubifs/dir.c                 |  65 +++++++++++-----
 fs/udf/dir.c                   |  28 ++++++-
 fs/ufs/dir.c                   |  28 ++++++-
 include/linux/fs.h             |  14 +++-
 20 files changed, 366 insertions(+), 122 deletions(-)
---
base-commit: 79868ff5ce9156228f9aef351d8bf76fb8540230
change-id: 20240829-vfs-file-f_version-66321e3dafeb