Date: Sun, 10 Mar 2024 21:50:51 +0000 From: Kirk McKusick <mckusick@freebsd.org> To: Konstantin Belousov <kostikbel@gmail.com> Cc: current@freebsd.org, Mark Millard <marklmi@yahoo.com> Subject: Re: Reason why "nocache" option is not displayed in "mount"? Message-ID: <20240310215051.EC14296B7@freefall.freebsd.org> In-Reply-To: <Ze3sMj6jf3upY3_G@kib.kiev.ua>
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> Date: Sun, 10 Mar 2024 19:21:54 +0200 > From: Konstantin Belousov <kostikbel@gmail.com> > To: Kirk McKusick <mckusick@mckusick.com> > Cc: current@freebsd.org > Subject: 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. As you note there are some filesystem specific flags already in mnt_flag that get copied to the statfs f_flags field. My point is that the NOCACHE flag could be moved to mnt_flag and made visible in the f_flags field. While it is currently specific to nullfs, it might be useful to implement it in other filesystems. Kirk McKusick
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