Date:	Tue, 13 Jul 2010 12:49:10 +0800
From:	Ian Kent <raven@...maw.net>
To:	Valerie Aurora <vaurora@...hat.com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	Miklos Szeredi <miklos@...redi.hu>,
	Jan Blunck <jblunck@...e.de>,
	Christoph Hellwig <hch@...radead.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 22/38] union-mount: Implement union lookup

On Tue, Jun 15, 2010 at 11:39:52AM -0700, Valerie Aurora wrote:
> Implement unioned directories, whiteouts, and fallthrus in pathname
> lookup routines.  do_lookup() and lookup_hash() call lookup_union()
> after looking up the dentry from the top-level file system.
> lookup_union() is centered around __lookup_hash(), which does cached
> and/or real lookups and revalidates each dentry in the union stack.
> 
> XXX - implement negative union cache entries
> 
> XXX - What about different permissions on different layers on the same
> directory name?  Should complain, fail, test permissions on all
> layers, what?
> ---
>  fs/namei.c |  171 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>  fs/union.c |   94 +++++++++++++++++++++++++++++++++
>  fs/union.h |    7 +++
>  3 files changed, 271 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index 06aad7e..45be5e5 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -35,6 +35,7 @@
>  #include <asm/uaccess.h>
>  
>  #include "internal.h"
> +#include "union.h"
>  
>  /* [Feb-1997 T. Schoebel-Theuer]
>   * Fundamental changes in the pathname lookup mechanisms (namei)
> @@ -722,6 +723,160 @@ static __always_inline void follow_dotdot(struct nameidata *nd)
>  	follow_mount(&nd->path);
>  }
>  
> +static struct dentry *__lookup_hash(struct qstr *name, struct dentry *base,
> +				    struct nameidata *nd);
> +
> +/*
> + * __lookup_union - Given a path from the topmost layer, lookup and
> + * revalidate each dentry in its union stack, building it if necessary
> + *
> + * @nd - nameidata for the parent of @topmost
> + * @name - pathname from this element on
> + * @topmost - path of the topmost matching dentry
> + *
> + * Given the nameidata and the path of the topmost dentry for this
> + * pathname, lookup, revalidate, and build the associated union stack.
> + * @topmost must be either a negative dentry or a directory, and not a
> + * whiteout.
> + *
> + * This function may stomp nd->path with the path of the parent
> + * directory of lower layer, so the caller must save nd->path and
> + * restore it afterwards.  You probably want to use lookup_union(),
> + * not __lookup_union().
> + */
> +
> +static int __lookup_union(struct nameidata *nd, struct qstr *name,
> +			  struct path *topmost)
> +{
> +	struct path parent = nd->path;
> +	struct path lower, upper;
> +	struct union_dir *ud;
> +	/* new_ud is the tail of the list of union dirs for this dentry */

Should new_ud be next_ud, since there is no new_ud defined?

> +	struct union_dir **next_ud = &topmost->dentry->d_union_dir;
> +	int err = 0;
> +
> +	/*
> +	 * upper is either a negative dentry from the top layer, or it
> +	 * is the most recent positive dentry for a directory that
> +	 * we've seen.
> +	 */
> +	upper = *topmost;
> +
> +	/* Go through each dir underlying the parent, looking for a match */
> +	for (ud = nd->path.dentry->d_union_dir; ud != NULL; ud = ud->u_lower) {
> +		BUG_ON(ud->u_this.dentry->d_count.counter == 0);
> +		/* Change the nameidata to point to this level's dir */
> +		nd->path = ud->u_this;
> +		/* Lookup the child in this level */
> +		lower.mnt = mntget(nd->path.mnt);
> +		mutex_lock(&nd->path.dentry->d_inode->i_mutex);
> +		lower.dentry = __lookup_hash(name, nd->path.dentry, nd);
> +		mutex_unlock(&nd->path.dentry->d_inode->i_mutex);
> +
> +		if (IS_ERR(lower.dentry)) {
> +			mntput(lower.mnt);
> +			err = PTR_ERR(lower.dentry);
> +			goto out;
> +		}
> +
> +		if (!lower.dentry->d_inode) {
> +			if (d_is_whiteout(lower.dentry))
> +				break;
> +			if (IS_OPAQUE(nd->path.dentry->d_inode) &&
> +			    !d_is_fallthru(lower.dentry))
> +				break;
> +			/* Plain old negative! Keep looking */
> +			path_put(&lower);
> +			continue;
> +		}
> +
> +		/* Finding a non-dir ends the lookup, one way or another */
> +		if (!S_ISDIR(lower.dentry->d_inode->i_mode)) {
> +			/* Ignore file below dir - invalid */
> +			if (upper.dentry->d_inode &&
> +			    S_ISDIR(upper.dentry->d_inode->i_mode)) {
> +				path_put(&lower);
> +				break;
> +			}
> +			/* Bingo, found our target */
> +			dput(topmost->dentry);
> +			/* mntput(topmost) done in link_path_walk() */
> +			*topmost = lower;
> +			break;
> +		}
> +
> +		/* Found a directory. Create the topmost version if it doesn't exist */
> +		if (!topmost->dentry->d_inode) {
> +			err = union_create_topmost_dir(&parent, name, topmost,
> +						       &lower);
> +			if (err) {
> +				path_put(&lower);
> +				return err;
> +			}
> +		}
> +
> +		err = union_add_dir(&upper, &lower, next_ud);
> +		if (err)
> +			break;
> +
> +		next_ud = &(*next_ud)->u_lower;
> +		upper = lower;
> +	}
> +out:
> +	return 0;
> +}
> +
> +/*
> + * lookup_union - revalidate and build union stack for this path
> + *
> + * We borrow the nameidata struct from the topmost layer to do the
> + * revalidation on lower dentries, replacing the topmost parent
> + * directory's path with that of the matching parent dir in each lower
> + * layer.  This wrapper for __lookup_union() saves the topmost layer's
> + * path and restores it when we are done.
> + */
> +static int lookup_union(struct nameidata *nd, struct qstr *name,
> +			struct path *topmost)
> +{
> +	struct path saved_path;
> +	int err;
> +
> +	BUG_ON(!IS_MNT_UNION(nd->path.mnt) && !IS_MNT_UNION(topmost->mnt));
> +	BUG_ON(!mutex_is_locked(&nd->path.dentry->d_inode->i_mutex));
> +
> +	saved_path = nd->path;
> +	path_get(&saved_path);
> +
> +	err = __lookup_union(nd, name, topmost);
> +
> +	nd->path = saved_path;
> +	path_put(&saved_path);
> +
> +	return err;
> +}
> +
> +/*
> + * do_union_lookup - union mount-aware part of do_lookup
> + *
> + * do_lookup()-style wrapper for lookup_union().  Follows mounts.
> + */
> +
> +static int do_lookup_union(struct nameidata *nd, struct qstr *name,
> +			   struct path *topmost)
> +{
> +	struct dentry *parent = nd->path.dentry;
> +	struct inode *dir = parent->d_inode;
> +	int err;
> +
> +	mutex_lock(&dir->i_mutex);
> +	err = lookup_union(nd, name, topmost);
> +	mutex_unlock(&dir->i_mutex);
> +
> +	__follow_mount(topmost);
> +
> +	return err;
> +}
> +
>  /*
>   *  It's more convoluted than I'd like it to be, but... it's still fairly
>   *  small and for now I'd prefer to have fast path as straight as possible.
> @@ -752,6 +907,11 @@ done:
>  	path->mnt = mnt;
>  	path->dentry = dentry;
>  	__follow_mount(path);
> +	if (needs_lookup_union(&nd->path, path)) {
> +		int err = do_lookup_union(nd, name, path);
> +		if (err < 0)
> +			return err;
> +	}
>  	return 0;
>  
>  need_lookup:
> @@ -1223,8 +1383,13 @@ static int lookup_hash(struct nameidata *nd, struct qstr *name,
>  		err = PTR_ERR(path->dentry);
>  		path->dentry = NULL;
>  		path->mnt = NULL;
> +		return err;
>  	}
> +
> +	if (needs_lookup_union(&nd->path, path))
> +		err = lookup_union(nd, name, path);
>  	return err;
> +
>  }
>  
>  static int __lookup_one_len(const char *name, struct qstr *this,
> @@ -2888,7 +3053,11 @@ SYSCALL_DEFINE4(renameat, int, olddfd, const char __user *, oldname,
>  	error = -EXDEV;
>  	if (oldnd.path.mnt != newnd.path.mnt)
>  		goto exit2;
> -
> +	/* Rename on union mounts not implemented yet */
> +	/* XXX much harsher check than necessary - can do some renames */
> +	if (IS_DIR_UNIONED(oldnd.path.dentry) ||
> +	    IS_DIR_UNIONED(newnd.path.dentry))
> +		goto exit2;
>  	old_dir = oldnd.path.dentry;
>  	error = -EBUSY;
>  	if (oldnd.last_type != LAST_NORM)
> diff --git a/fs/union.c b/fs/union.c
> index 02abb7c..c089c02 100644
> --- a/fs/union.c
> +++ b/fs/union.c
> @@ -21,6 +21,7 @@
>  #include <linux/mount.h>
>  #include <linux/fs_struct.h>
>  #include <linux/slab.h>
> +#include <linux/namei.h>
>  
>  #include "union.h"
>  
> @@ -117,3 +118,96 @@ void d_free_unions(struct dentry *dentry)
>  	}
>  	dentry->d_union_dir = NULL;
>  }
> +
> +/**
> + * needs_lookup_union - Avoid union lookup when not necessary
> + *
> + * @parent_path: path of the parent directory
> + * @path: path of the lookup target
> + *
> + * Check to see if the target needs union lookup.  Two cases need
> + * union lookup: the target is a directory, and the target is a
> + * negative dentry.
> + *
> + * Returns 0 if this dentry is definitely not unioned.  Returns 1 if
> + * it is possible this dentry is unioned.
> + */
> +
> +int needs_lookup_union(struct path *parent_path, struct path *path)
> +{
> +	/*
> +	 * If the target is the root of the mount, then its union
> +	 * stack was already created at mount time (if this is a union
> +	 * mount).
> +	 */
> +	if (IS_ROOT(path->dentry))
> +		return 0;
> +
> +	/* Only dentries in a unioned directory need a union lookup. */
> +	if (!IS_DIR_UNIONED(parent_path->dentry))
> +		return 0;
> +
> +	/* Whiteouts cover up everything below */
> +	if (d_is_whiteout(path->dentry))
> +		return 0;
> +
> +	/* Opaque dirs cover except if this is a fallthru */
> +	if (IS_OPAQUE(parent_path->dentry->d_inode) &&
> +	    !d_is_fallthru(path->dentry))
> +		return 0;
> +
> +	/*
> +	 * XXX Negative dentries in unioned directories must always go
> +	 * through a full union lookup because there might be a
> +	 * matching entry below it.  To improve performance, we should
> +	 * mark negative dentries in some way to show they have
> +	 * already been looked up in the union and nothing was found.
> +	 * Maybe mark it opaque?
> +	 */
> +	if (!path->dentry->d_inode)
> +		return 1;
> +
> +	/*
> +	 * If it's not a directory and it's a positive dentry, then we
> +	 * already have the topmost dentry and we don't need to do any
> +	 * lookup in lower layers.
> +	 */
> +
> +	if (!S_ISDIR(path->dentry->d_inode->i_mode))
> +		return 0;
> +
> +	/* Is the union stack already constructed? */
> +	if (IS_DIR_UNIONED(path->dentry))
> +		return 0;
> +
> +	/*
> +	 * XXX This is like the negative dentry case.  This directory
> +	 * may have no matching directories in the lower layers, or
> +	 * this may just be the first time we looked it up.  We can't
> +	 * tell the difference.
> +	 */
> +	return 1;
> +}
> +
> +/*
> + * union_create_topmost_dir - Create a matching dir in the topmost file system
> + */
> +
> +int union_create_topmost_dir(struct path *parent, struct qstr *name,
> +			     struct path *topmost, struct path *lower)
> +{
> +	int mode = lower->dentry->d_inode->i_mode;
> +	int res;
> +
> +	BUG_ON(topmost->dentry->d_inode);
> +
> +	res = mnt_want_write(parent->mnt);
> +	if (res)
> +		return res;
> +
> +	res = vfs_mkdir(parent->dentry->d_inode, topmost->dentry, mode);
> +
> +	mnt_drop_write(parent->mnt);
> +
> +	return res;
> +}
> diff --git a/fs/union.h b/fs/union.h
> index 04efc1f..505f132 100644
> --- a/fs/union.h
> +++ b/fs/union.h
> @@ -51,15 +51,22 @@ struct union_dir {
>  };
>  
>  #define IS_MNT_UNION(mnt)	((mnt)->mnt_flags & MNT_UNION)
> +#define IS_DIR_UNIONED(dentry)	((dentry)->d_union_dir)
>  
>  extern int union_add_dir(struct path *, struct path *, struct union_dir **);
>  extern void d_free_unions(struct dentry *);
> +int needs_lookup_union(struct path *, struct path *);
> +int union_create_topmost_dir(struct path *, struct qstr *, struct path *,
> +			     struct path *);
>  
>  #else /* CONFIG_UNION_MOUNT */
>  
>  #define IS_MNT_UNION(x)			(0)
> +#define IS_DIR_UNIONED(x)		(0)
>  #define union_add_dir(x, y, z)		({ BUG(); (NULL); })
>  #define d_free_unions(x)		do { } while (0)
> +#define needs_lookup_union(x, y)	({ (0); })
> +#define union_create_topmost_dir(w, x, y, z)	({ BUG(); (NULL); })
>  
>  #endif	/* CONFIG_UNION_MOUNT */
>  #endif	/* __KERNEL__ */
> -- 
> 1.6.3.3
> 
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