Re: Reason why "nocache" option is not displayed in "mount"?

On Sun, Mar 10, 2024 at 01:53:05AM +0000, Kirk McKusick wrote:
> The issue has to do with how flags are defined in mount.h.
> Specifically there are the flags that are externally visible
> (prefixed with MNT_) and those that are for internal use
> (prefixed with MNTK_, the K standing for KERNEL). If it
> is desirable to have MNTK_NULL_NOCACHE visible, then it
> should be renamed to MNT_NULL_CACHE, added to MNT_VISFLAGMASK,
> and listed in MNTOPT_NAMES. It probably belongs in the set
> described as `Flags set by internal operations, but visible
> to the user.' With this change, it will be displayed by
> the mount command and show up in the statfs flags.

There is no MNTK_NULL_NOCACHE flag in mnt_kern_flags.

When userspace communicates the "cache" or "nocache" option to the
VFS_MOUNT() op for nullfs, it passes plain C string using the nmount(2)
system call. The strings are explicitly queried by nullfs_mount(), mixed
with the "default" sysctl, and then the nullfs-mount specific data flag
is set, in mp->mnt_data.null_flag.

There is no space in the struct statfs for ABI extension.
The getfsstat(2) system call cannot report arbitrary fs-specific options.

If somebody wants to uniformilly report fs-specific options, instead of
scattered fs-specific hacks like MNT_SOFTDEP/MNT_GJOURNAL (UFS) and
nfsstat -m (nfsclient), then some extension for nmount(2) is due,
say MNT_QUERY_OP, which should be passed down to VFS_MOUNT() and back.