Message-ID: <5524C556.1040607@cn.fujitsu.com>
Date:	Wed, 8 Apr 2015 14:06:14 +0800
From:	Qu Wenruo <quwenruo@...fujitsu.com>
To:	Omar Sandoval <osandov@...ndov.com>, Chris Mason <clm@...com>,
	Josef Bacik <jbacik@...com>, David Sterba <dsterba@...e.cz>,
	<linux-btrfs@...r.kernel.org>
CC:	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] Btrfs: unify subvol= and subvolid= mounting



-------- Original Message  --------
Subject: [PATCH 2/3] Btrfs: unify subvol= and subvolid= mounting
From: Omar Sandoval <osandov@...ndov.com>
To: Chris Mason <clm@...com>, Josef Bacik <jbacik@...com>, David Sterba 
<dsterba@...e.cz>, <linux-btrfs@...r.kernel.org>
Date: 2015年04月08日 13:34

> Currently, mounting a subvolume with subvolid= takes a different code
> path than mounting with subvol=. This isn't really a big deal except for
> the fact that mounts done with subvolid= or the default subvolume don't
> have a dentry that's connected to the dentry tree like in the subvol=
> case. To unify the code paths, when given subvolid= or using the default
> subvolume ID, translate it into a subvolume name by walking
> ROOT_BACKREFs in the root tree and INODE_REFs in the filesystem trees.
Oh, this patch is what I have tried long long ago, and want to do the 
same thing, to show subvolume mount for btrfs.

But it came to me that, superblock->show_path() is a better method to do it.

You can implement btrfs_show_path() to allow mountinfo to get the 
subvolume name from subvolid, and don't change the mount routine much.

Thanks,
Qu
>
> Signed-off-by: Omar Sandoval <osandov@...ndov.com>
> ---
>   fs/btrfs/super.c | 347 ++++++++++++++++++++++++++++++++++++-------------------
>   1 file changed, 225 insertions(+), 122 deletions(-)
>
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index d38be09..5ab9801 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -841,33 +841,153 @@ out:
>   	return error;
>   }
>
> -static struct dentry *get_default_root(struct super_block *sb,
> -				       u64 subvol_objectid)
> +static char *get_subvol_name_from_objectid(struct btrfs_fs_info *fs_info,
> +					   u64 subvol_objectid)
>   {
> -	struct btrfs_fs_info *fs_info = btrfs_sb(sb);
>   	struct btrfs_root *root = fs_info->tree_root;
> -	struct btrfs_root *new_root;
> -	struct btrfs_dir_item *di;
> -	struct btrfs_path *path;
> -	struct btrfs_key location;
> -	struct inode *inode;
> -	u64 dir_id;
> -	int new = 0;
> +	struct btrfs_root *fs_root;
> +	struct btrfs_root_ref *root_ref;
> +	struct btrfs_inode_ref *inode_ref;
> +	struct btrfs_key key;
> +	struct btrfs_path *path = NULL;
> +	char *name = NULL, *ptr;
> +	u64 dirid;
> +	int len;
> +	int ret;
> +
> +	path = btrfs_alloc_path();
> +	if (!path) {
> +		ret = -ENOMEM;
> +		goto err;
> +	}
> +	path->leave_spinning = 1;
> +
> +	name = kmalloc(PATH_MAX, GFP_NOFS);
> +	if (!name) {
> +		ret = -ENOMEM;
> +		goto err;
> +	}
> +	ptr = name + PATH_MAX - 1;
> +	ptr[0] = '\0';
>
>   	/*
> -	 * We have a specific subvol we want to mount, just setup location and
> -	 * go look up the root.
> +	 * Walk up the subvolume trees in the tree of tree roots by root
> +	 * backrefs until we hit the top-level subvolume.
>   	 */
> -	if (subvol_objectid) {
> -		location.objectid = subvol_objectid;
> -		location.type = BTRFS_ROOT_ITEM_KEY;
> -		location.offset = (u64)-1;
> -		goto find_root;
> +	while (subvol_objectid != BTRFS_FS_TREE_OBJECTID) {
> +		key.objectid = subvol_objectid;
> +		key.type = BTRFS_ROOT_BACKREF_KEY;
> +		key.offset = (u64)-1;
> +
> +		ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
> +		if (ret < 0) {
> +			goto err;
> +		} else if (ret > 0) {
> +			ret = btrfs_previous_item(root, path, subvol_objectid,
> +						  BTRFS_ROOT_BACKREF_KEY);
> +			if (ret < 0) {
> +				goto err;
> +			} else if (ret > 0) {
> +				ret = -ENOENT;
> +				goto err;
> +			}
> +		}
> +
> +		btrfs_item_key_to_cpu(path->nodes[0], &key, path->slots[0]);
> +		subvol_objectid = key.offset;
> +
> +		root_ref = btrfs_item_ptr(path->nodes[0], path->slots[0],
> +					  struct btrfs_root_ref);
> +		len = btrfs_root_ref_name_len(path->nodes[0], root_ref);
> +		ptr -= len + 1;
> +		if (ptr < name) {
> +			ret = -ENAMETOOLONG;
> +			goto err;
> +		}
> +		read_extent_buffer(path->nodes[0], ptr + 1,
> +				   (unsigned long)(root_ref + 1), len);
> +		ptr[0] = '/';
> +		dirid = btrfs_root_ref_dirid(path->nodes[0], root_ref);
> +		btrfs_release_path(path);
> +
> +		key.objectid = subvol_objectid;
> +		key.type = BTRFS_ROOT_ITEM_KEY;
> +		key.offset = (u64)-1;
> +		fs_root = btrfs_read_fs_root_no_name(fs_info, &key);
> +		if (IS_ERR(fs_root)) {
> +			ret = PTR_ERR(fs_root);
> +			goto err;
> +		}
> +
> +		/*
> +		 * Walk up the filesystem tree by inode refs until we hit the
> +		 * root directory.
> +		 */
> +		while (dirid != BTRFS_FIRST_FREE_OBJECTID) {
> +			key.objectid = dirid;
> +			key.type = BTRFS_INODE_REF_KEY;
> +			key.offset = (u64)-1;
> +
> +			ret = btrfs_search_slot(NULL, fs_root, &key, path, 0, 0);
> +			if (ret < 0) {
> +				goto err;
> +			} else if (ret > 0) {
> +				ret = btrfs_previous_item(fs_root, path, dirid,
> +							  BTRFS_INODE_REF_KEY);
> +				if (ret < 0) {
> +					goto err;
> +				} else if (ret > 0) {
> +					ret = -ENOENT;
> +					goto err;
> +				}
> +			}
> +
> +			btrfs_item_key_to_cpu(path->nodes[0], &key, path->slots[0]);
> +			dirid = key.offset;
> +
> +			inode_ref = btrfs_item_ptr(path->nodes[0],
> +						   path->slots[0],
> +						   struct btrfs_inode_ref);
> +			len = btrfs_inode_ref_name_len(path->nodes[0],
> +						       inode_ref);
> +			ptr -= len + 1;
> +			if (ptr < name) {
> +				ret = -ENAMETOOLONG;
> +				goto err;
> +			}
> +			read_extent_buffer(path->nodes[0], ptr + 1,
> +					   (unsigned long)(inode_ref + 1), len);
> +			ptr[0] = '/';
> +			btrfs_release_path(path);
> +		}
> +	}
> +
> +	btrfs_free_path(path);
> +	if (ptr == name + PATH_MAX - 1) {
> +		name[0] = '/';
> +		name[1] = '\0';
> +	} else {
> +		memmove(name, ptr, name + PATH_MAX - ptr);
>   	}
> +	return name;
> +
> +err:
> +	btrfs_free_path(path);
> +	kfree(name);
> +	return ERR_PTR(ret);
> +}
> +
> +static int get_default_subvol_objectid(struct btrfs_fs_info *fs_info, u64 *objectid)
> +{
> +	struct btrfs_root *root = fs_info->tree_root;
> +	struct btrfs_dir_item *di;
> +	struct btrfs_path *path;
> +	struct btrfs_key location;
> +	u64 dir_id;
>
>   	path = btrfs_alloc_path();
>   	if (!path)
> -		return ERR_PTR(-ENOMEM);
> +		return -ENOMEM;
>   	path->leave_spinning = 1;
>
>   	/*
> @@ -879,49 +999,23 @@ static struct dentry *get_default_root(struct super_block *sb,
>   	di = btrfs_lookup_dir_item(NULL, root, path, dir_id, "default", 7, 0);
>   	if (IS_ERR(di)) {
>   		btrfs_free_path(path);
> -		return ERR_CAST(di);
> +		return PTR_ERR(di);
>   	}
>   	if (!di) {
>   		/*
>   		 * Ok the default dir item isn't there.  This is weird since
>   		 * it's always been there, but don't freak out, just try and
> -		 * mount to root most subvolume.
> +		 * mount the top-level subvolume.
>   		 */
>   		btrfs_free_path(path);
> -		dir_id = BTRFS_FIRST_FREE_OBJECTID;
> -		new_root = fs_info->fs_root;
> -		goto setup_root;
> +		*objectid = BTRFS_FS_TREE_OBJECTID;
> +		return 0;
>   	}
>
>   	btrfs_dir_item_key_to_cpu(path->nodes[0], di, &location);
>   	btrfs_free_path(path);
> -
> -find_root:
> -	new_root = btrfs_read_fs_root_no_name(fs_info, &location);
> -	if (IS_ERR(new_root))
> -		return ERR_CAST(new_root);
> -
> -	dir_id = btrfs_root_dirid(&new_root->root_item);
> -setup_root:
> -	location.objectid = dir_id;
> -	location.type = BTRFS_INODE_ITEM_KEY;
> -	location.offset = 0;
> -
> -	inode = btrfs_iget(sb, &location, new_root, &new);
> -	if (IS_ERR(inode))
> -		return ERR_CAST(inode);
> -
> -	/*
> -	 * If we're just mounting the root most subvol put the inode and return
> -	 * a reference to the dentry.  We will have already gotten a reference
> -	 * to the inode in btrfs_fill_super so we're good to go.
> -	 */
> -	if (!new && sb->s_root->d_inode == inode) {
> -		iput(inode);
> -		return dget(sb->s_root);
> -	}
> -
> -	return d_obtain_root(inode);
> +	*objectid = location.objectid;
> +	return 0;
>   }
>
>   static int btrfs_fill_super(struct super_block *sb,
> @@ -1129,109 +1223,123 @@ static inline int is_subvolume_inode(struct inode *inode)
>   }
>
>   /*
> - * This will strip out the subvol=%s argument for an argument string and add
> - * subvolid=0 to make sure we get the actual tree root for path walking to the
> - * subvol we want.
> + * This will add subvolid=0 to the argument string while removing any subvol=
> + * and subvolid= arguments to make sure we get the top-level root for path
> + * walking to the subvol we want.
>    */
>   static char *setup_root_args(char *args)
>   {
> -	unsigned len = strlen(args) + 2 + 1;
> -	char *src, *dst, *buf;
> -
> -	/*
> -	 * We need the same args as before, but with this substitution:
> -	 * s!subvol=[^,]+!subvolid=0!
> -	 *
> -	 * Since the replacement string is up to 2 bytes longer than the
> -	 * original, allocate strlen(args) + 2 + 1 bytes.
> -	 */
> +	char *p, *dst, *buf;
>
> -	src = strstr(args, "subvol=");
> -	/* This shouldn't happen, but just in case.. */
> -	if (!src)
> -		return NULL;
> +	if (!args)
> +		return kstrdup("subvolid=0", GFP_NOFS);
>
> -	buf = dst = kmalloc(len, GFP_NOFS);
> +	/* The worst case is that we add ",subvolid=0" to the end. */
> +	buf = dst = kmalloc(strlen(args) + strlen(",subvolid=0") + 1, GFP_NOFS);
>   	if (!buf)
>   		return NULL;
>
> -	/*
> -	 * If the subvol= arg is not at the start of the string,
> -	 * copy whatever precedes it into buf.
> -	 */
> -	if (src != args) {
> -		*src++ = '\0';
> -		strcpy(buf, args);
> -		dst += strlen(args);
> +	while (1) {
> +		p = strchrnul(args, ',');
> +		if (strncmp(args, "subvol=", strlen("subvol=")) != 0 &&
> +		    strncmp(args, "subvolid=", strlen("subvolid=")) != 0) {
> +			memcpy(dst, args, p - args);
> +			dst += p - args;
> +			*dst++ = ',';
> +		}
> +		if (*p)
> +			args = p + 1;
> +		else
> +			break;
>   	}
> -
>   	strcpy(dst, "subvolid=0");
> -	dst += strlen("subvolid=0");
> -
> -	/*
> -	 * If there is a "," after the original subvol=... string,
> -	 * copy that suffix into our buffer.  Otherwise, we're done.
> -	 */
> -	src = strchr(src, ',');
> -	if (src)
> -		strcpy(dst, src);
>
>   	return buf;
>   }
>
> -static struct dentry *mount_subvol(const char *subvol_name, int flags,
> -				   const char *device_name, char *data)
> +static struct dentry *mount_subvol(const char *subvol_name, u64 subvol_objectid,
> +				   int flags, const char *device_name,
> +				   char *data)
>   {
>   	struct dentry *root;
> -	struct vfsmount *mnt;
> +	struct vfsmount *mnt = NULL;
>   	char *newargs;
> +	int ret;
>
>   	newargs = setup_root_args(data);
> -	if (!newargs)
> -		return ERR_PTR(-ENOMEM);
> -	mnt = vfs_kern_mount(&btrfs_fs_type, flags, device_name,
> -			     newargs);
> +	if (!newargs) {
> +		root = ERR_PTR(-ENOMEM);
> +		goto out;
> +	}
>
> -	if (PTR_RET(mnt) == -EBUSY) {
> +	mnt = vfs_kern_mount(&btrfs_fs_type, flags, device_name, newargs);
> +	if (PTR_ERR_OR_ZERO(mnt) == -EBUSY) {
>   		if (flags & MS_RDONLY) {
> -			mnt = vfs_kern_mount(&btrfs_fs_type, flags & ~MS_RDONLY, device_name,
> -					     newargs);
> +			mnt = vfs_kern_mount(&btrfs_fs_type, flags & ~MS_RDONLY,
> +					     device_name, newargs);
>   		} else {
> -			int r;
> -			mnt = vfs_kern_mount(&btrfs_fs_type, flags | MS_RDONLY, device_name,
> -					     newargs);
> +			mnt = vfs_kern_mount(&btrfs_fs_type, flags | MS_RDONLY,
> +					     device_name, newargs);
>   			if (IS_ERR(mnt)) {
> -				kfree(newargs);
> -				return ERR_CAST(mnt);
> +				root = ERR_CAST(mnt);
> +				mnt = NULL;
> +				goto out;
>   			}
>
>   			down_write(&mnt->mnt_sb->s_umount);
> -			r = btrfs_remount(mnt->mnt_sb, &flags, NULL);
> +			ret = btrfs_remount(mnt->mnt_sb, &flags, NULL);
>   			up_write(&mnt->mnt_sb->s_umount);
> -			if (r < 0) {
> -				/* FIXME: release vfsmount mnt ??*/
> -				kfree(newargs);
> -				return ERR_PTR(r);
> +			if (ret < 0) {
> +				root = ERR_PTR(ret);
> +				goto out;
>   			}
>   		}
>   	}
> +	if (IS_ERR(mnt)) {
> +		root = ERR_CAST(mnt);
> +		mnt = NULL;
> +		goto out;
> +	}
>
> -	kfree(newargs);
> +	if (!subvol_name) {
> +		if (!subvol_objectid) {
> +			ret = get_default_subvol_objectid(btrfs_sb(mnt->mnt_sb),
> +							  &subvol_objectid);
> +			if (ret) {
> +				root = ERR_PTR(ret);
> +				goto out;
> +			}
> +		}
> +		subvol_name = get_subvol_name_from_objectid(btrfs_sb(mnt->mnt_sb),
> +							    subvol_objectid);
> +		if (IS_ERR(subvol_name)) {
> +			root = ERR_CAST(subvol_name);
> +			subvol_name = NULL;
> +			goto out;
> +		}
>
> -	if (IS_ERR(mnt))
> -		return ERR_CAST(mnt);
> +	}
>
>   	root = mount_subtree(mnt, subvol_name);
> +	mnt = NULL; /* mount_subtree drops our reference on the vfsmount. */
>
>   	if (!IS_ERR(root) && !is_subvolume_inode(root->d_inode)) {
>   		struct super_block *s = root->d_sb;
>   		dput(root);
>   		root = ERR_PTR(-EINVAL);
>   		deactivate_locked_super(s);
> -		printk(KERN_ERR "BTRFS: '%s' is not a valid subvolume\n",
> -				subvol_name);
> +		pr_err("BTRFS: '%s' is not a valid subvolume\n", subvol_name);
> +	}
> +	if (!IS_ERR(root) && subvol_objectid &&
> +	    BTRFS_I(root->d_inode)->root->root_key.objectid != subvol_objectid) {
> +		pr_warn("BTRFS: subvol '%s' does not match subvolid %llu\n",
> +			subvol_name, subvol_objectid);
>   	}
>
> +out:
> +	mntput(mnt);
> +	kfree(newargs);
> +	kfree(subvol_name);
>   	return root;
>   }
>
> @@ -1296,7 +1404,6 @@ static struct dentry *btrfs_mount(struct file_system_type *fs_type, int flags,
>   {
>   	struct block_device *bdev = NULL;
>   	struct super_block *s;
> -	struct dentry *root;
>   	struct btrfs_fs_devices *fs_devices = NULL;
>   	struct btrfs_fs_info *fs_info = NULL;
>   	struct security_mnt_opts new_sec_opts;
> @@ -1316,10 +1423,10 @@ static struct dentry *btrfs_mount(struct file_system_type *fs_type, int flags,
>   		return ERR_PTR(error);
>   	}
>
> -	if (subvol_name) {
> -		root = mount_subvol(subvol_name, flags, device_name, data);
> -		kfree(subvol_name);
> -		return root;
> +	if (subvol_name || subvol_objectid != BTRFS_FS_TREE_OBJECTID) {
> +		/* mount_subvol() will free subvol_name. */
> +		return mount_subvol(subvol_name, subvol_objectid, flags,
> +				    device_name, data);
>   	}
>
>   	security_init_mnt_opts(&new_sec_opts);
> @@ -1385,23 +1492,19 @@ static struct dentry *btrfs_mount(struct file_system_type *fs_type, int flags,
>   		error = btrfs_fill_super(s, fs_devices, data,
>   					 flags & MS_SILENT ? 1 : 0);
>   	}
> -
> -	root = !error ? get_default_root(s, subvol_objectid) : ERR_PTR(error);
> -	if (IS_ERR(root)) {
> +	if (error) {
>   		deactivate_locked_super(s);
> -		error = PTR_ERR(root);
>   		goto error_sec_opts;
>   	}
>
>   	fs_info = btrfs_sb(s);
>   	error = setup_security_options(fs_info, s, &new_sec_opts);
>   	if (error) {
> -		dput(root);
>   		deactivate_locked_super(s);
>   		goto error_sec_opts;
>   	}
>
> -	return root;
> +	return dget(s->s_root);
>
>   error_close_devices:
>   	btrfs_close_devices(fs_devices);
>
--
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