linux-kernel - Re: [PATCH RFC 1/3] fs: introduce helper d_path

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Date:   Sat, 8 May 2021 21:05:44 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Jia He <justin.he@....com>, Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Jonathan Corbet <corbet@....net>,
        Al Viro <viro@....linux.org.uk>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Eric Biggers <ebiggers@...gle.com>,
        "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH RFC 1/3] fs: introduce helper d_path_fast()

On Sat, May 08, 2021 at 01:39:45PM -0700, Linus Torvalds wrote:

> +static inline int prepend_entries(struct prepend_buffer *b, const struct path *path, const struct path *root, struct mount *mnt)

If anything, s/path/dentry/, since vfsmnt here will be equal to &mnt->mnt all along.

> +{
> +	struct dentry *dentry = path->dentry;
> +	struct vfsmount *vfsmnt = path->mnt;
> +
> +	while (dentry != root->dentry || vfsmnt != root->mnt) {
> +		int error;
> +		struct dentry * parent;
> +
> +		if (dentry == vfsmnt->mnt_root || IS_ROOT(dentry)) {
> +			struct mount *parent = READ_ONCE(mnt->mnt_parent);
> +			struct mnt_namespace *mnt_ns;
> +
> +			/* Escaped? */
> +			if (dentry != vfsmnt->mnt_root)
> +				return 3;
> +
> +			/* Global root? */
> +			if (mnt != parent) {
> +				dentry = READ_ONCE(mnt->mnt_mountpoint);
> +				mnt = parent;
> +				vfsmnt = &mnt->mnt;
> +				continue;
> +			}
> +			mnt_ns = READ_ONCE(mnt->mnt_ns);
> +			/* open-coded is_mounted() to use local mnt_ns */
> +			if (!IS_ERR_OR_NULL(mnt_ns) && !is_anon_ns(mnt_ns))
> +				return 1;	// absolute root
> +
> +			return 2;		// detached or not attached yet
> +			break;

?

> +		}
> +		parent = dentry->d_parent;
> +		prefetch(parent);
> +		error = prepend_name(b, &dentry->d_name);
> +		if (error)
> +			break;

return error, surely?

> +		dentry = parent;
> +	}
> +	return 0;
> +}

FWIW, if we go that way, I would make that

	while (dentry != root->dentry || &mnt->mnt != root->mnt) {
		int error;
		struct dentry *parent = READ_ONCE(dentry->d_parent);

		if (unlikely(dentry == mnt->mnt.mnt_root)) {
			struct mount *m = READ_ONCE(mnt->mnt_parent);

			/* Global root? */
			if (unlikely(mnt == m) {
				struct mnt_namespace *mnt_ns;

				mnt_ns = READ_ONCE(mnt->mnt_ns);
				/* open-coded is_mounted() to use local mnt_ns */
				if (!IS_ERR_OR_NULL(mnt_ns) && !is_anon_ns(mnt_ns))
					return 1;	// absolute root
				else
					return 2;	// detached or not attached yet
			}
			// cross into whatever it's mounted on
			dentry = READ_ONCE(mnt->mnt_mountpoint);
			mnt = m;
			continue;
		}

		/* Escaped? */
		if (unlikely(dentry == parent))
			return 3;

		prefetch(parent);
		error = prepend_name(b, &dentry->d_name);
		if (error)
			return error;
		dentry = parent;
	}
	return 0;