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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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