[PATCH v2 0/3] Check errors on sync for volatile overlayfs mounts

[Sargun Dhillon <sargun@sargun.me>]

The semantics of errseq and syncfs are such that it is impossible to track
if any errors have occurred between the time the first error occurred, and
the user checks for the error (calls syncfs, and subsequently
errseq_check_and_advance.

Overlayfs has a volatile feature which short-circuits syncfs. This, in turn
makes it so that the user can have silent data corruption and not know
about it. The third patch in the series introduces behaviour that makes it
so that we can track errors, and bubble up whether the user has put
themselves in bad situation.

This required some gymanstics in errseq, and adding a wrapper around it
called "errseq_counter" (errseq + counter). The data structure uses an
atomic to track overflow errors. This approach, rather than moving to an
atomic64 / u64 is so we can avoid bloating every person that subscribes to
an errseq, and only add the subscriber behaviour to those who care (at the
expense of space.

The datastructure is write-optimized, and rightfully so, as the users
of the counter feature are just overlayfs, and it's called in fsync
checking, which is a rather seldom operation, and not really on
any hotpaths.

[1]: https://lore.kernel.org/linux-fsdevel/20201202092720.41522-1-sargun@sargun.me/

Sargun Dhillon (3):
  errseq: Add errseq_counter to allow for all errors to be observed
  errseq: Add mechanism to snapshot errseq_counter and check snapshot
  overlay: Implement volatile-specific fsync error behaviour

 Documentation/filesystems/overlayfs.rst |   8 ++
 fs/buffer.c                             |   2 +-
 fs/overlayfs/file.c                     |   5 +-
 fs/overlayfs/overlayfs.h                |   1 +
 fs/overlayfs/ovl_entry.h                |   3 +
 fs/overlayfs/readdir.c                  |   5 +-
 fs/overlayfs/super.c                    |  26 +++--
 fs/overlayfs/util.c                     |  28 +++++
 fs/super.c                              |   1 +
 fs/sync.c                               |   3 +-
 include/linux/errseq.h                  |  18 ++++
 include/linux/fs.h                      |   6 +-
 include/linux/pagemap.h                 |   2 +-
 lib/errseq.c                            | 129 ++++++++++++++++++++----
 14 files changed, 202 insertions(+), 35 deletions(-)

-- 
2.25.1