[PATCH v5 0/8] fs: implement multigrain timestamps

[Jeff Layton]

The VFS always uses coarse-grained timestamps when updating the
ctime and mtime after a change. This has the benefit of allowing
filesystems to optimize away a lot metadata updates, down to around 1
per jiffy, even when a file is under heavy writes.

Unfortunately, this coarseness has always been an issue when we're
exporting via NFSv3, which relies on timestamps to validate caches. A
lot of changes can happen in a jiffy, so timestamps aren't sufficient to
help the client decide to invalidate the cache.

Even with NFSv4, a lot of exported filesystems don't properly support a
change attribute and are subject to the same problems with timestamp
granularity. Other applications have similar issues with timestamps (e.g
backup applications).

If we were to always use fine-grained timestamps, that would improve the
situation, but that becomes rather expensive, as the underlying
filesystem would have to log a lot more metadata updates.

What we need is a way to only use fine-grained timestamps when they are
being actively queried. The idea is to use an unused bit in the ctime's
tv_nsec field to mark when the mtime or ctime has been queried via
getattr. Once that has been marked, the next m/ctime update will use a
fine-grained timestamp.

This patch series is based on top of Christian's vfs.all branch, which
has the recent conversion to the new ctime accessors. It should apply
cleanly on top of linux-next.

While the patchset does work, I'm mostly looking for feedback on the
core infrastructure API. Does this look reasonable? Am I missing any
races?

Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
base-commit: cf22d118b89a09a0160586412160d89098f7c4c7
---
Jeff Layton (8):
      fs: pass the request_mask to generic_fillattr
      fs: add infrastructure for multigrain timestamps
      tmpfs: bump the mtime/ctime/iversion when page becomes writeable
      tmpfs: add support for multigrain timestamps
      xfs: XFS_ICHGTIME_CREATE is unused
      xfs: switch to multigrain timestamps
      ext4: switch to multigrain timestamps
      btrfs: convert to multigrain timestamps

 fs/9p/vfs_inode.c               |  4 +-
 fs/9p/vfs_inode_dotl.c          |  4 +-
 fs/afs/inode.c                  |  2 +-
 fs/btrfs/file.c                 | 24 ++--------
 fs/btrfs/inode.c                |  2 +-
 fs/btrfs/super.c                |  5 ++-
 fs/ceph/inode.c                 |  2 +-
 fs/coda/inode.c                 |  3 +-
 fs/ecryptfs/inode.c             |  5 ++-
 fs/erofs/inode.c                |  2 +-
 fs/exfat/file.c                 |  2 +-
 fs/ext2/inode.c                 |  2 +-
 fs/ext4/inode.c                 |  2 +-
 fs/ext4/super.c                 |  2 +-
 fs/f2fs/file.c                  |  2 +-
 fs/fat/file.c                   |  2 +-
 fs/fuse/dir.c                   |  2 +-
 fs/gfs2/inode.c                 |  2 +-
 fs/hfsplus/inode.c              |  2 +-
 fs/inode.c                      | 98 +++++++++++++++++++++++++++++------------
 fs/kernfs/inode.c               |  2 +-
 fs/libfs.c                      |  4 +-
 fs/minix/inode.c                |  2 +-
 fs/nfs/inode.c                  |  2 +-
 fs/nfs/namespace.c              |  3 +-
 fs/ntfs3/file.c                 |  2 +-
 fs/ocfs2/file.c                 |  2 +-
 fs/orangefs/inode.c             |  2 +-
 fs/proc/base.c                  |  4 +-
 fs/proc/fd.c                    |  2 +-
 fs/proc/generic.c               |  2 +-
 fs/proc/proc_net.c              |  2 +-
 fs/proc/proc_sysctl.c           |  2 +-
 fs/proc/root.c                  |  3 +-
 fs/smb/client/inode.c           |  2 +-
 fs/smb/server/smb2pdu.c         | 22 ++++-----
 fs/smb/server/vfs.c             |  3 +-
 fs/stat.c                       | 59 ++++++++++++++++++++-----
 fs/sysv/itree.c                 |  3 +-
 fs/ubifs/dir.c                  |  2 +-
 fs/udf/symlink.c                |  2 +-
 fs/vboxsf/utils.c               |  2 +-
 fs/xfs/libxfs/xfs_shared.h      |  2 -
 fs/xfs/libxfs/xfs_trans_inode.c |  8 ++--
 fs/xfs/xfs_iops.c               |  4 +-
 fs/xfs/xfs_super.c              |  2 +-
 include/linux/fs.h              | 47 ++++++++++++++++++--
 mm/shmem.c                      | 16 ++++++-
 48 files changed, 248 insertions(+), 129 deletions(-)
---
base-commit: cf22d118b89a09a0160586412160d89098f7c4c7
change-id: 20230713-mgctime-f2a9fc324918

Best regards,
-- 
Jeff Layton <jlayton@xxxxxxxxxx>