(Apologies for the resend, but I didn't send this with a wide enough distribution list originally). A few weeks ago, during one of the discussions around i_version, Dave Chinner wrote this: "You've missed the part where I suggested lifting the "nfsd sampled i_version" state into an inode state flag rather than hiding it in the i_version field. At that point, we could optimise away the secondary ctime updates just like you are proposing we do with the i_version updates. Further, we could also use that state it to decide whether we need to use high resolution timestamps when recording ctime updates - if the nfsd has not sampled the ctime/i_version, we don't need high res timestamps to be recorded for ctime...." While I don't think we can practically optimize away ctime updates like we do with i_version, I do like the idea of using this scheme to indicate when we need to use a high-res timestamp. This patchset is a first stab at a scheme to do this. It declares a new i_state flag for this purpose and adds two new vfs-layer functions to implement conditional high-res timestamp fetching. It then converts both tmpfs and xfs to use it. This seems to behave fine under xfstests, but I haven't yet done any performance testing with it. I wouldn't expect it to create huge regressions though since we're only grabbing high res timestamps after each query. I like this scheme because we can potentially convert any filesystem to use it. No special storage requirements like with i_version field. I think it'd potentially improve NFS cache coherency with a whole swath of exportable filesystems, and helps out NFSv3 too. This is really just a proof-of-concept. There are a number of things we could change: 1/ We could use the top bit in the tv_sec field as the flag. That'd give us different flags for ctime and mtime. We also wouldn't need to use a spinlock. 2/ We could probably optimize away the high-res timestamp fetch in more cases. Basically, always do a coarse-grained ts fetch and only fetch the high-res ts when the QUERIED flag is set and the existing time hasn't changed. If this approach looks reasonable, I'll plan to start working on converting more filesystems. One thing I'm not clear on is how widely available high res timestamps are. Is this something we need to gate on particular CONFIG_* options? Thoughts? Jeff Layton (3): fs: add infrastructure for opportunistic high-res ctime/mtime updates shmem: mark for high-res timestamps on next update after getattr xfs: mark the inode for high-res timestamp update in getattr fs/inode.c | 40 +++++++++++++++++++++++++++++++-- fs/stat.c | 10 +++++++++ fs/xfs/libxfs/xfs_trans_inode.c | 2 +- fs/xfs/xfs_acl.c | 2 +- fs/xfs/xfs_inode.c | 2 +- fs/xfs/xfs_iops.c | 15 ++++++++++--- include/linux/fs.h | 5 ++++- mm/shmem.c | 23 ++++++++++--------- 8 files changed, 80 insertions(+), 19 deletions(-) -- 2.39.2
