On Mon, Apr 24, 2023 at 11:11:02AM -0400, Jeff Layton wrote: > The VFS always uses coarse-grained timestamp updates for filling out the > ctime and mtime after a change. This has the benefit of allowing > filesystems to optimize away a lot metaupdates, to around once per > jiffy, even when a file is under heavy writes. > > Unfortunately, this has always been an issue when we're exporting via > NFSv3, which relies on timestamps to validate caches. 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 (e.g backup applications). > > Switching to always using fine-grained timestamps would improve the > situation for NFS, but that becomes rather expensive, as the underlying > filesystem will 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: > > Whenever the mtime changes, the ctime must also change since we're > changing the metadata. When a superblock has a s_time_gran >1, we can > use the lowest-order bit of the inode->i_ctime as a flag to indicate > that the value has been queried. Then on the next write, we'll fetch a > fine-grained timestamp instead of the usual coarse-grained one. > > We could enable this for any filesystem that has a s_time_gran >1, but > for now, this patch adds a new SB_MULTIGRAIN_TS flag to allow filesystems > to opt-in to this behavior. Hm, the patch raises the flag in s_flags. Please at least move this to s_iflags as SB_I_MULTIGRAIN and treat this as an internal flag. There's no need to give the impression that this will become a mount option. Also, this looks like it's a filesystem property not a superblock property as the granularity isn't changeable. So shouldn't this be an FS_* flag instead?
