Re: [PATCH v2 1/3] fs: add infrastructure for multigrain inode i_m/ctime

[Christian Brauner <brauner@kernel.org>]

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?