[PATCH v2 0/3] fat: add support for the renameat2 RENAME_EXCHANGE flag

[Javier Martinez Canillas]

Hello,

This series add support for the renameat2 system call RENAME_EXCHANGE flag
(which allows to atomically replace two paths) to the vfat filesystem code.

There are many use cases for this, but we are particularly interested in
making possible for vfat filesystems to be part of OSTree [0] deployments.

Currently OSTree relies on symbolic links to make the deployment updates
an atomic transactional operation. But RENAME_EXCHANGE could be used [1]
to achieve a similar level of robustness when using a vfat filesystem.

Patch #1 is just a preparatory patch to introduce the RENAME_EXCHANGE
support in patch #2 and finally patch #3 adds some kselftests to test it.

This is a v2 that addresses issues pointed out in the first version that
was posted as an RFC:

https://lore.kernel.org/lkml/20220519092343.2776414-1-javierm@xxxxxxxxxx/

[0]: https://github.com/ostreedev/ostree
[1]: https://github.com/ostreedev/ostree/issues/1649

Changes in v2:
- Only update the new_dir inode version and timestamps if != old_dir
  (Alex Larsson).
- Add some helper functions to avoid duplicating code (OGAWA Hirofumi).
- Use braces for multi-lines blocks even if are one statement (OGAWA Hirofumi).
- Mention in commit message that the operation is as transactional as possible
  but within the vfat limitations of not having a journal (Colin Walters).
- Call sync to flush the page cache before checking the file contents
  (Alex Larsson).
- Drop RFC prefix since the patches already got some review.

Javier Martinez Canillas (3):
  fat: add a vfat_rename2() and make existing .rename callback a helper
  fat: add renameat2 RENAME_EXCHANGE flag support
  selftests/filesystems: add a vfat RENAME_EXCHANGE test

 MAINTAINERS                                   |   1 +
 fs/fat/namei_vfat.c                           | 193 +++++++++++++++++-
 tools/testing/selftests/Makefile              |   1 +
 .../selftests/filesystems/fat/Makefile        |   7 +
 .../testing/selftests/filesystems/fat/config  |   2 +
 .../filesystems/fat/rename_exchange.c         |  37 ++++
 .../filesystems/fat/run_fat_tests.sh          |  82 ++++++++
 7 files changed, 316 insertions(+), 7 deletions(-)
 create mode 100644 tools/testing/selftests/filesystems/fat/Makefile
 create mode 100644 tools/testing/selftests/filesystems/fat/config
 create mode 100644 tools/testing/selftests/filesystems/fat/rename_exchange.c
 create mode 100755 tools/testing/selftests/filesystems/fat/run_fat_tests.sh

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2.36.1