linux-kernel - Some filesystems set MNT_* flags in superblock->s

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Message-ID: <19326.1496994530@warthog.procyon.org.uk>
Date:   Fri, 09 Jun 2017 08:48:50 +0100
From:   David Howells <dhowells@...hat.com>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     dhowells@...hat.com, mszeredi@...hat.com, jlayton@...hat.com,
        linux-fsdevel@...r.kernel.org, linux-nfs@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Some filesystems set MNT_* flags in superblock->s_flags

Al Viro <viro@...IV.linux.org.uk> wrote:

> 	* what the hell is ms_flags thing doing in __vfs_new_sb_config()?
> It's a really vile mix of unrelated flags and operations we had in existing
> mount(2) ABI.  With MS_KERNMOUNT thrown into that loo^Wmix.  Sure, we need
> to parse the garbage fed to mount(2).  And we need to pass that garbage to
> "legacy" types as well, but let's not inflict it upon the new mechanisms.

Hmmm...  Some ->remount_fs() operations attempt to alter the MS_* flags that
correspond to MNT_* flags.  Coda, for example:

	static int coda_remount(struct super_block *sb, int *flags, char *data)
	{
		sync_filesystem(sb);
		*flags |= MS_NOATIME;
		return 0;
	}

But this is quashed in do_remount_sb:

	sb->s_flags = (sb->s_flags & ~MS_RMT_MASK) | (flags & MS_RMT_MASK);

And others set them directly in s_flags, v9fs_fill_super() for example sets
MS_NOATIME:

	sb->s_flags |= MS_ACTIVE | MS_DIRSYNC | MS_NOATIME;

I'm guessing things like this should be got rid of, but does there need to be
a way to inform mount() that these should be set on the vfsmount?

David