fs: rework and optimize i_version handling in filesystems

From: Jeff Layton <[email protected]>

v5:
- don't corrupt refcounts stashed in i_version of ext4 xattr inodes
- add raw variants of inc and cmp functions, and have nfs use them

v4:
- fix SB_LAZYTIME handling in generic_update_time
- add memory barriers to patch to convert i_version field to atomic64_t

v3:
- move i_version handling functions to new header file
- document that the kernel-managed i_version implementation will appear to increase over time
- fix inode_cmp_iversion to handle wraparound correctly

v2:
- xfs should use inode_peek_iversion instead of inode_peek_iversion_raw
- rework file_update_time patch
- don't dirty inode when only S_ATIME is set and SB_LAZYTIME is enabled
- better comments and documentation

I think this is now approaching merge readiness.

Special thanks to Jan Kara and Dave Chinner who helped me tighten up the memory barriers in the final patch, and Krzysztof Kozlowski for help in tracking down a set of bugs in the NFS client patch.

tl;dr: I think we can greatly reduce the cost of the inode->i_version counter, by exploiting the fact that we don't need to increment it if no one is looking at it. We can also clean up the code to prepare to eventually expose this value via statx().

Note that this set relies on a few patches that are in other trees. The full stack that I've been testing with is here:

https://git.kernel.org/pub/scm/linux/kernel/git/jlayton/linux.git/log/?h=iversion

The inode->i_version field is supposed to be a value that changes whenever there is any data or metadata change to the inode. Some filesystems use it internally to detect directory changes during readdir. knfsd will use it if the filesystem has MS_I_VERSION set. IMA will also use it to optimize away some remeasurement if it's available. NFS and AFS just use it to store an opaque change attribute from the server.

Only btrfs, ext4, and xfs increment it for data changes. Because of this, these filesystems must log the inode to disk whenever the i_version counter changes. That has a non-zero performance impact, especially on write-heavy workloads, because we end up dirtying the inode metadata on every write, not just when the times change.

It turns out though that none of these users of i_version require that it change on every change to the file. The only real requirement is that it be different if something changed since the last time we queried for it.

If we keep track of when something queries the value, we can avoid bumping the counter and an on-disk update when nothing else has changed if no one has queried it since it was last incremented.

This patchset changes the code to only bump the i_version counter when it's strictly necessary, or when we're updating the inode metadata anyway (e.g. when times change).

It takes the approach of converting the existing accessors of i_version to use a new API, while leaving the underlying implementation mostly the same. The last patch then converts the existing implementation to keep track of whether the value has been queried since it was last incremented. It then uses that to avoid incrementing the counter when it can.

With this, we reduce inode metadata updates across all 3 filesystems down to roughly the frequency of the timestamp granularity, particularly when it's not being queried (the vastly common case).

I can see measurable performance gains on xfs and ext4 with iversion enabled, when streaming small (4k) I/Os.

btrfs shows some slight gain in testing, but not quite the magnitude that xfs and ext4 show. I'm not sure why yet and would appreciate some input from btrfs folks.

My goal is to get this into linux-next fairly soon. If it shows no problems then we can look at merging it for 4.16, or 4.17 if all of the prequisite patches are not yet merged.

Jeff Layton (19):
fs: new API for handling inode->i_version
fs: don't take the i_lock in inode_inc_iversion
fat: convert to new i_version API
affs: convert to new i_version API
afs: convert to new i_version API
btrfs: convert to new i_version API
exofs: switch to new i_version API
ext2: convert to new i_version API
ext4: convert to new i_version API
nfs: convert to new i_version API
nfsd: convert to new i_version API
ocfs2: convert to new i_version API
ufs: use new i_version API
xfs: convert to new i_version API
IMA: switch IMA over to new i_version API
fs: only set S_VERSION when updating times if necessary
xfs: avoid setting XFS_ILOG_CORE if i_version doesn't need
incrementing
btrfs: only dirty the inode in btrfs_update_time if something was
changed
fs: handle inode->i_version more efficiently

 fs/affs/amigaffs.c                |   5 +-
 fs/affs/dir.c                     |   5 +-
 fs/affs/super.c                   |   3 +-
 fs/afs/fsclient.c                 |   3 +-
 fs/afs/inode.c                    |   5 +-
 fs/btrfs/delayed-inode.c          |   7 +-
 fs/btrfs/file.c                   |   1 +
 fs/btrfs/inode.c                  |  12 +-
 fs/btrfs/ioctl.c                  |   1 +
 fs/btrfs/tree-log.c               |   4 +-
 fs/btrfs/xattr.c                  |   1 +
 fs/exofs/dir.c                    |   9 +-
 fs/exofs/super.c                  |   3 +-
 fs/ext2/dir.c                     |   9 +-
 fs/ext2/super.c                   |   5 +-
 fs/ext4/dir.c                     |   9 +-
 fs/ext4/inline.c                  |   7 +-
 fs/ext4/inode.c                   |  13 +-
 fs/ext4/ioctl.c                   |   3 +-
 fs/ext4/namei.c                   |   5 +-
 fs/ext4/super.c                   |   3 +-
 fs/ext4/xattr.c                   |   5 +-
 fs/fat/dir.c                      |   3 +-
 fs/fat/inode.c                    |   9 +-
 fs/fat/namei_msdos.c              |   7 +-
 fs/fat/namei_vfat.c               |  22 +--
 fs/inode.c                        |  11 +-
 fs/nfs/delegation.c               |   3 +-
 fs/nfs/fscache-index.c            |   5 +-
 fs/nfs/inode.c                    |  18 +-
 fs/nfs/nfs4proc.c                 |  10 +-
 fs/nfs/nfstrace.h                 |   5 +-
 fs/nfs/write.c                    |   8 +-
 fs/nfsd/nfsfh.h                   |   3 +-
 fs/ocfs2/dir.c                    |  15 +-
 fs/ocfs2/inode.c                  |   3 +-
 fs/ocfs2/namei.c                  |   3 +-
 fs/ocfs2/quota_global.c           |   3 +-
 fs/ufs/dir.c                      |   9 +-
 fs/ufs/inode.c                    |   3 +-
 fs/ufs/super.c                    |   3 +-
 fs/xfs/libxfs/xfs_inode_buf.c     |   7 +-
 fs/xfs/xfs_icache.c               |   5 +-
 fs/xfs/xfs_inode.c                |   3 +-
 fs/xfs/xfs_inode_item.c           |   3 +-
 fs/xfs/xfs_trans_inode.c          |  16 +-
 include/linux/fs.h                |  17 +-
 include/linux/iversion.h          | 335 ++++++++++++++++++++++++++++++++++++++
 security/integrity/ima/ima_api.c  |   3 +-
 security/integrity/ima/ima_main.c |   3 +-
 50 files changed, 518 insertions(+), 135 deletions(-)
 create mode 100644 include/linux/iversion.h

2.14.3