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    Subject[PATCH] cowlinks v2
    Hi!

    Version 2 of my cowlink patch, tested and currently running on my
    machine.

    Al, I'd especially like your opinion on it. Would you accept
    something like this?

    Changes since v1:
    o moved break_cow_link() check to get_write_access()
    o added inode locking when changing flags
    o switched to mark_inode_dirty_sync()

    TODO:
    o Disallow fcntl() for filesystems without support
    o Proper support for ext[23]
    o Switch to mark_inode_dirty() without sync?
    o Library support for
    o copyfile() (link and set cow-flag)
    o cow_open() (break link if open() fails)

    Jörn

    --
    Premature optimization is the root of all evil.
    -- Donald Knuth


    fs/ext2/inode.c | 3 +-
    fs/ext3/inode.c | 4 ++
    fs/fcntl.c | 21 +++++++++++++++
    fs/namei.c | 70 ++++++++++++++++++++++++++++++++++++++++++++++++++
    include/linux/fcntl.h | 3 ++
    include/linux/fs.h | 3 ++
    6 files changed, 102 insertions(+), 2 deletions(-)

    --- linux-2.6.4/include/linux/fcntl.h~cowlink 2004-03-19 17:38:48.000000000 +0100
    +++ linux-2.6.4/include/linux/fcntl.h 2004-03-19 17:52:49.000000000 +0100
    @@ -23,6 +23,9 @@
    #define DN_ATTRIB 0x00000020 /* File changed attibutes */
    #define DN_MULTISHOT 0x80000000 /* Don't remove notifier */

    +#define F_SETCOW (F_LINUX_SPECIFIC_BASE+3)
    +#define F_GETCOW (F_LINUX_SPECIFIC_BASE+4)
    +
    #ifdef __KERNEL__

    #if BITS_PER_LONG == 32
    --- linux-2.6.4/include/linux/fs.h~cowlink 2004-03-19 17:47:29.000000000 +0100
    +++ linux-2.6.4/include/linux/fs.h 2004-03-19 17:52:49.000000000 +0100
    @@ -137,6 +137,9 @@
    #define S_DEAD 32 /* removed, but still open directory */
    #define S_NOQUOTA 64 /* Inode is not counted to quota */
    #define S_DIRSYNC 128 /* Directory modifications are synchronous */
    +#define S_COWLINK 256 /* Hard links have copy on write semantics.
    + * This flag has no meaning for directories,
    + * but is inherited to directory children */

    /*
    * Note that nosuid etc flags are inode-specific: setting some file-system
    --- linux-2.6.4/fs/fcntl.c~cowlink 2004-03-19 17:47:15.000000000 +0100
    +++ linux-2.6.4/fs/fcntl.c 2004-03-19 17:59:20.000000000 +0100
    @@ -282,6 +282,20 @@

    EXPORT_SYMBOL(f_delown);

    +static long fcntl_setcow(struct file *filp, unsigned long arg)
    +{
    + struct inode *inode = filp->f_dentry->d_inode;
    +
    + spin_lock(&inode->i_lock);
    + if (arg)
    + inode->i_flags |= S_COWLINK;
    + else
    + inode->i_flags &= ~S_COWLINK;
    + mark_inode_dirty_sync(inode);
    + spin_unlock(&inode->i_lock);
    + return 0;
    +}
    +
    static long do_fcntl(unsigned int fd, unsigned int cmd,
    unsigned long arg, struct file * filp)
    {
    @@ -346,6 +360,13 @@
    case F_NOTIFY:
    err = fcntl_dirnotify(fd, filp, arg);
    break;
    + case F_SETCOW:
    + err = fcntl_setcow(filp, arg);
    + break;
    + case F_GETCOW:
    + err = (filp->f_dentry->d_inode->i_flags & S_COWLINK) /
    + S_COWLINK;
    + break;
    default:
    break;
    }
    --- linux-2.6.4/fs/namei.c~cowlink 2004-03-19 17:47:19.000000000 +0100
    +++ linux-2.6.4/fs/namei.c 2004-03-19 18:10:00.000000000 +0100
    @@ -224,6 +224,33 @@
    }

    /*
    + * Files with the S_COWLINK flag set cannot be written to, if more
    + * than one hard link to them exists. Ultimately, this function
    + * should copy the inode, assign the copy to the dentry and lower use
    + * count of the old inode - one day.
    + * For now, it is sufficient to return an error and let userspace
    + * deal with the messy part. Not exactly the meaning of
    + * copy-on-write, but much better than writing to fifty files at once
    + * and noticing month later.
    + *
    + * Yes, this breaks the kernel interface and is simply wrong. This
    + * is intended behaviour, so Linus will not merge the code before
    + * it is complete. Or will he?
    + */
    +static int break_cow_link(struct inode *inode)
    +{
    + if (!(inode->i_flags & S_COWLINK))
    + return 0;
    + if (!S_ISREG(inode->i_mode))
    + return 0;
    + if (inode->i_nlink < 2)
    + return 0;
    + /* TODO: As soon as sendfile can do normal file copies, use that
    + * and always return 0 */
    + return -EMLINK;
    +}
    +
    +/*
    * get_write_access() gets write permission for a file.
    * put_write_access() releases this write permission.
    * This is used for regular files.
    @@ -243,7 +270,14 @@

    int get_write_access(struct inode * inode)
    {
    + int error;
    +
    spin_lock(&inode->i_lock);
    + error = break_cow_link(inode);
    + if (error) {
    + spin_unlock(&inode->i_lock);
    + return error;
    + }
    if (atomic_read(&inode->i_writecount) < 0) {
    spin_unlock(&inode->i_lock);
    return -ETXTBSY;
    @@ -1148,6 +1182,10 @@
    if (!error) {
    inode_dir_notify(dir, DN_CREATE);
    security_inode_post_create(dir, dentry, mode);
    + spin_lock(&inode->i_lock);
    + dentry->d_inode->i_flags |= dir->i_flags & S_COWLINK;
    + mark_inode_dirty_sync(inode);
    + spin_unlock(&inode->i_lock);
    }
    return error;
    }
    @@ -1522,6 +1560,9 @@
    if (!error) {
    inode_dir_notify(dir, DN_CREATE);
    security_inode_post_mkdir(dir,dentry, mode);
    + spin_lock(&inode->i_lock);
    + dentry->d_inode->i_flags |= dir->i_flags & S_COWLINK;
    + spin_unlock(&inode->i_lock);
    }
    return error;
    }
    @@ -1820,6 +1861,13 @@
    return -EXDEV;

    /*
    + * Cowlink attribute is inherited from directory, but here,
    + * the inode already has one. If they don't match, bail out.
    + */
    + if ((dir->i_flags ^ old_dentry->d_inode->i_flags) & S_COWLINK)
    + return -EMLINK;
    +
    + /*
    * A link to an append-only or immutable file cannot be created.
    */
    if (IS_APPEND(inode) || IS_IMMUTABLE(inode))
    @@ -1997,6 +2045,24 @@
    return error;
    }

    +static int cow_allow_rename(struct inode *old_dir, struct dentry *old_dentry,
    + struct inode *new_dir)
    +{
    + /* source and target share directory: allow */
    + if (old_dir == new_dir)
    + return 0;
    + /* source and target directory have identical cowlink flag: allow */
    + if (! ((old_dentry->d_inode->i_flags ^ new_dir->i_flags) & S_COWLINK))
    + return 0;
    + /* We could always fail here, but cowlink flag is only defined for
    + * files and directories, so let's allow special files */
    + if (!S_ISREG(old_dentry->d_inode->i_mode))
    + return -EMLINK;
    + if (!S_ISDIR(old_dentry->d_inode->i_mode))
    + return -EMLINK;
    + return 0;
    +}
    +
    int vfs_rename(struct inode *old_dir, struct dentry *old_dentry,
    struct inode *new_dir, struct dentry *new_dentry)
    {
    @@ -2020,6 +2086,10 @@
    if (!old_dir->i_op || !old_dir->i_op->rename)
    return -EPERM;

    + error = cow_allow_rename(old_dir, old_dentry, new_dir);
    + if (error)
    + return error;
    +
    DQUOT_INIT(old_dir);
    DQUOT_INIT(new_dir);

    --- linux-2.6.4/fs/ext2/inode.c~cowlink 2004-03-19 17:44:02.000000000 +0100
    +++ linux-2.6.4/fs/ext2/inode.c 2004-03-19 17:52:49.000000000 +0100
    @@ -1020,6 +1020,7 @@
    {
    unsigned int flags = EXT2_I(inode)->i_flags;

    + inode->i_flags = flags;
    inode->i_flags &= ~(S_SYNC|S_APPEND|S_IMMUTABLE|S_NOATIME|S_DIRSYNC);
    if (flags & EXT2_SYNC_FL)
    inode->i_flags |= S_SYNC;
    @@ -1191,7 +1192,7 @@

    raw_inode->i_blocks = cpu_to_le32(inode->i_blocks);
    raw_inode->i_dtime = cpu_to_le32(ei->i_dtime);
    - raw_inode->i_flags = cpu_to_le32(ei->i_flags);
    + raw_inode->i_flags = cpu_to_le32(inode->i_flags);
    raw_inode->i_faddr = cpu_to_le32(ei->i_faddr);
    raw_inode->i_frag = ei->i_frag_no;
    raw_inode->i_fsize = ei->i_frag_size;
    --- linux-2.6.4/fs/ext3/inode.c~cowlink 2004-03-19 17:44:02.000000000 +0100
    +++ linux-2.6.4/fs/ext3/inode.c 2004-03-19 17:52:49.000000000 +0100
    @@ -2447,6 +2447,7 @@
    {
    unsigned int flags = EXT3_I(inode)->i_flags;

    + inode->i_flags = flags;
    inode->i_flags &= ~(S_SYNC|S_APPEND|S_IMMUTABLE|S_NOATIME|S_DIRSYNC);
    if (flags & EXT3_SYNC_FL)
    inode->i_flags |= S_SYNC;
    @@ -2629,7 +2630,8 @@
    raw_inode->i_mtime = cpu_to_le32(inode->i_mtime.tv_sec);
    raw_inode->i_blocks = cpu_to_le32(inode->i_blocks);
    raw_inode->i_dtime = cpu_to_le32(ei->i_dtime);
    - raw_inode->i_flags = cpu_to_le32(ei->i_flags);
    + raw_inode->i_flags = cpu_to_le32((ei->i_flags & ~S_COWLINK) |
    + (inode->i_flags & S_COWLINK));
    #ifdef EXT3_FRAGMENTS
    raw_inode->i_faddr = cpu_to_le32(ei->i_faddr);
    raw_inode->i_frag = ei->i_frag_no;
    -
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