[PATCH COW] MAD COW

From: Jörn Engel
Date: Thu May 06 2004 - 08:37:44 EST

Patch 5

Jörn

--
A victorious army first wins and then seeks battle.
-- Sun Tzu

Allow COW behaviour for hard links, depending on an inode flag.

Semantics:
o Files with S_COWLINK do cow. (yes, really ;)
o Directories with S_COWLINK inherit flag to new files.
o If in doubt, return -EMLINK and let the user sort it out:
- When linking non-cow files to cow directories.
- When moving non-cow files/directories to cow directories.
- When moving cow files/directories to non-cow directories.

Thanks to Sytse and Andrew for tips.

fs/ext2/inode.c | 11 ++++++
fs/ext2/super.c | 2 -
fs/ext3/inode.c | 11 ++++++
fs/ext3/super.c | 2 -
fs/fcntl.c | 24 ++++++++++++++
fs/namei.c | 76 +++++++++++++++++++++++++++++++++++++++++++++-
fs/open.c | 79 +++++++++++++++++++++++++++++++++++++++++++++---
include/linux/ext2_fs.h | 1
include/linux/ext3_fs.h | 1
include/linux/fcntl.h | 3 +
include/linux/fs.h | 4 ++
11 files changed, 205 insertions(+), 9 deletions(-)


--- linux-2.6.5cow/fs/open.c~break_madcow 2004-05-04 16:31:50.000000000 +0200
+++ linux-2.6.5cow/fs/open.c 2004-05-06 01:02:57.000000000 +0200
@@ -723,6 +723,71 @@
return error;
}

+char *index(const char *s, int c)
+{
+ while (*s) {
+ if (*s == c)
+ return (char *) s;
+ s++;
+ }
+ return NULL;
+}
+
+char *rindex(const char *s, int c)
+{
+ char *ret = NULL;
+ while (*s) {
+ if (*s == c)
+ ret = (char *) s;
+ s++;
+ }
+ return ret;
+}
+
+char *madcow_temp_name(const char *name)
+{
+ const char temp[] = "__MADCOW_BREAK_LINK"; /* FIXME: add random part */
+ char *last_slash = rindex(name, '/');
+ size_t dir_len = last_slash ? last_slash + 1 - name : 0;
+ size_t len = sizeof(temp) + dir_len;
+ char *ret;
+
+ ret = kmalloc(len, GFP_KERNEL);
+ if (!ret)
+ return NULL;
+
+ strncpy(ret, name, dir_len);
+ strcpy(ret+dir_len, temp);
+ return ret;
+}
+
+int vfs_rename_other(struct inode *, struct dentry *,
+ struct inode *, struct dentry *);
+int do_copyfile(const char *from, const char *to, int mode);
+int do_rename(const char * oldname, const char * newname);
+
+int madcow_break_link(const char *from)
+{
+ int err, ret = -EMLINK;
+ char *to = madcow_temp_name(from);
+
+ printk("break link '%s' -> '%s'\n", from, to);
+ err = do_copyfile(from, to, -1);
+ printk("do_copyfile returned %d\n", err);
+ if (err)
+ goto out;
+
+ err = do_rename(to, from);
+ printk("do_rename returned %d\n", err);
+ if (err)
+ goto out;
+
+ ret = 0;
+out:
+ kfree(to);
+ return ret;
+}
+
/*
* Note that while the flag value (low two bits) for sys_open means:
* 00 - read-only
@@ -746,13 +811,19 @@
if ((namei_flags+1) & O_ACCMODE)
namei_flags++;
if (namei_flags & O_TRUNC)
- namei_flags |= 2;
+ namei_flags |= FMODE_WRITE;

error = open_namei(filename, namei_flags, mode, &nd);
- if (!error)
- return dentry_open(nd.dentry, nd.mnt, flags);

- return ERR_PTR(error);
+ if (error == -EMLINK) {
+ error = madcow_break_link(filename);
+ if (!error) /*retry*/
+ error = open_namei(filename, namei_flags, mode, &nd);
+ }
+ if (error)
+ return ERR_PTR(error);
+
+ return dentry_open(nd.dentry, nd.mnt, flags);
}

EXPORT_SYMBOL(filp_open);
--- linux-2.6.5cow/fs/ext2/inode.c~madcow 2004-04-27 16:48:55.000000000 +0200
+++ linux-2.6.5cow/fs/ext2/inode.c 2004-04-27 17:02:33.000000000 +0200
@@ -1021,7 +1021,8 @@
unsigned int flags = EXT2_I(inode)->i_flags;

spin_lock(inode->i_lock);
- inode->i_flags &= ~(S_SYNC|S_APPEND|S_IMMUTABLE|S_NOATIME|S_DIRSYNC);
+ inode->i_flags &= ~(S_SYNC | S_APPEND | S_IMMUTABLE | S_NOATIME
+ | S_DIRSYNC | S_MADCOWLINK);
if (flags & EXT2_SYNC_FL)
inode->i_flags |= S_SYNC;
if (flags & EXT2_APPEND_FL)
@@ -1032,6 +1033,8 @@
inode->i_flags |= S_NOATIME;
if (flags & EXT2_DIRSYNC_FL)
inode->i_flags |= S_DIRSYNC;
+ if (flags & EXT2_MADCOWLINK_FL)
+ inode->i_flags |= S_MADCOWLINK;
spin_unlock(inode->i_lock);
}

@@ -1159,6 +1162,12 @@
if (IS_ERR(raw_inode))
return -EIO;

+ /* vfs inode holds the current MADCOWLINK flag, so we have to update
+ * ei->i_flags first */
+ ei->i_flags &= ~EXT2_MADCOWLINK_FL;
+ if (inode_flags(inode, S_MADCOWLINK))
+ ei->i_flags |= EXT2_MADCOWLINK_FL;
+
/* For fields not not tracking in the in-memory inode,
* initialise them to zero for new inodes. */
if (ei->i_state & EXT2_STATE_NEW)
--- linux-2.6.5cow/fs/ext2/super.c~madcow 2004-04-27 16:34:42.000000000 +0200
+++ linux-2.6.5cow/fs/ext2/super.c 2004-04-27 17:03:10.000000000 +0200
@@ -1015,7 +1015,7 @@
.name = "ext2",
.get_sb = ext2_get_sb,
.kill_sb = kill_block_super,
- .fs_flags = FS_REQUIRES_DEV,
+ .fs_flags = FS_REQUIRES_DEV | FS_MADCOW,
};

static int __init init_ext2_fs(void)
--- linux-2.6.5cow/fs/ext3/inode.c~madcow 2004-04-27 16:48:55.000000000 +0200
+++ linux-2.6.5cow/fs/ext3/inode.c 2004-04-27 17:04:41.000000000 +0200
@@ -2448,7 +2448,8 @@
unsigned int flags = EXT3_I(inode)->i_flags;

spin_lock(inode->i_lock);
- inode->i_flags &= ~(S_SYNC|S_APPEND|S_IMMUTABLE|S_NOATIME|S_DIRSYNC);
+ inode->i_flags &= ~(S_SYNC | S_APPEND | S_IMMUTABLE | S_NOATIME
+ | S_DIRSYNC | S_MADCOWLINK);
if (flags & EXT3_SYNC_FL)
inode->i_flags |= S_SYNC;
if (flags & EXT3_APPEND_FL)
@@ -2459,6 +2460,8 @@
inode->i_flags |= S_NOATIME;
if (flags & EXT3_DIRSYNC_FL)
inode->i_flags |= S_DIRSYNC;
+ if (flags & EXT3_MADCOWLINK_FL)
+ inode->i_flags |= S_MADCOWLINK;
spin_unlock(inode->i_lock);
}

@@ -2594,6 +2597,12 @@
struct buffer_head *bh = iloc->bh;
int err = 0, rc, block;

+ /* vfs inode holds the current MADCOWLINK flag, so we have to update
+ * ei->i_flags first */
+ ei->i_flags &= ~EXT3_MADCOWLINK_FL;
+ if (inode_flags(inode, S_MADCOWLINK))
+ ei->i_flags |= EXT3_MADCOWLINK_FL;
+
/* For fields not not tracking in the in-memory inode,
* initialise them to zero for new inodes. */
if (ei->i_state & EXT3_STATE_NEW)
--- linux-2.6.5cow/fs/ext3/super.c~madcow 2004-04-27 16:34:42.000000000 +0200
+++ linux-2.6.5cow/fs/ext3/super.c 2004-04-27 17:05:22.000000000 +0200
@@ -2004,7 +2004,7 @@
.name = "ext3",
.get_sb = ext3_get_sb,
.kill_sb = kill_block_super,
- .fs_flags = FS_REQUIRES_DEV,
+ .fs_flags = FS_REQUIRES_DEV | FS_MADCOW,
};

static int __init init_ext3_fs(void)
--- linux-2.6.5cow/fs/fcntl.c~madcow 2004-04-27 16:34:42.000000000 +0200
+++ linux-2.6.5cow/fs/fcntl.c 2004-04-27 16:55:42.000000000 +0200
@@ -282,6 +282,24 @@

EXPORT_SYMBOL(f_delown);

+static long fcntl_setmadcow(struct file *filp, unsigned long arg)
+{
+ struct inode *inode = filp->f_dentry->d_inode;
+
+ if (!(inode->i_sb->s_type->fs_flags & FS_MADCOW))
+ return -EINVAL;
+ //FIXME: -EPERM?
+
+ spin_lock(&inode->i_lock);
+ if (arg)
+ inode->i_flags |= S_MADCOWLINK;
+ else
+ inode->i_flags &= ~S_MADCOWLINK;
+ spin_unlock(&inode->i_lock);
+ mark_inode_dirty(inode);
+ return 0;
+}
+
static long do_fcntl(unsigned int fd, unsigned int cmd,
unsigned long arg, struct file * filp)
{
@@ -346,6 +364,12 @@
case F_NOTIFY:
err = fcntl_dirnotify(fd, filp, arg);
break;
+ case F_SETMADCOW:
+ err = fcntl_setmadcow(filp, arg);
+ break;
+ case F_GETMADCOW:
+ err = !!inode_flags(filp->f_dentry->d_inode, S_MADCOWLINK);
+ break;
default:
break;
}
--- linux-2.6.5cow/fs/namei.c~madcow 2004-04-27 16:48:55.000000000 +0200
+++ linux-2.6.5cow/fs/namei.c 2004-05-02 15:07:57.000000000 +0200
@@ -223,6 +223,41 @@
return security_inode_permission(inode, mask, nd);
}

+static inline void set_madcowflag(struct inode *inode)
+{
+ spin_lock(&inode->i_lock);
+ inode->i_flags |= S_MADCOWLINK;
+ spin_unlock(&inode->i_lock);
+ mark_inode_dirty(inode);
+}
+
+/*
+ * Files with the S_MADCOWLINK 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_madcow_link(struct inode *inode)
+{
+ if (!inode_flags(inode, S_MADCOWLINK))
+ 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.
@@ -243,6 +278,10 @@

int get_write_access(struct inode * inode)
{
+ int error = break_madcow_link(inode);
+ if (error)
+ return error;
+
spin_lock(&inode->i_lock);
if (atomic_read(&inode->i_writecount) < 0) {
spin_unlock(&inode->i_lock);
@@ -1146,6 +1185,8 @@
DQUOT_INIT(dir);
error = dir->i_op->create(dir, dentry, mode, nd);
if (!error) {
+ if (inode_flags(dir, S_MADCOWLINK))
+ set_madcowflag(dentry->d_inode);
inode_dir_notify(dir, DN_CREATE);
security_inode_post_create(dir, dentry, mode);
}
@@ -1520,6 +1561,8 @@
DQUOT_INIT(dir);
error = dir->i_op->mkdir(dir, dentry, mode);
if (!error) {
+ if (inode_flags(dir, S_MADCOWLINK))
+ set_madcowflag(dentry->d_inode);
inode_dir_notify(dir, DN_CREATE);
security_inode_post_mkdir(dir,dentry, mode);
}
@@ -1823,6 +1866,13 @@
return -EXDEV;

/*
+ * Madcowlink attribute is inherited from directory, but here,
+ * the inode already has one. If they don't match, bail out.
+ */
+ if (inode_flags(dir, S_MADCOWLINK) != inode_flags(inode, S_MADCOWLINK))
+ return -EMLINK;
+
+ /*
* A link to an append-only or immutable file cannot be created.
*/
if (IS_APPEND(inode) || IS_IMMUTABLE(inode))
@@ -2003,6 +2053,26 @@
return error;
}

+static int madcow_allow_rename(struct inode *old_dir, struct dentry *old_dentry,
+ struct inode *new_dir)
+{
+ struct inode *old_inode = old_dentry->d_inode;
+
+ /* source and target share directory: allow */
+ if (old_dir == new_dir)
+ return 0;
+ /* source and target directory have identical madcowlink flag: allow */
+ if (inode_flags(old_inode, S_MADCOWLINK) == inode_flags(new_dir, S_MADCOWLINK))
+ return 0;
+ /* We could always fail here, but madcowlink flag is only defined for
+ * files and directories, so let's allow special files */
+ if (!S_ISREG(old_inode->i_mode))
+ return -EMLINK;
+ if (!S_ISDIR(old_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)
{
@@ -2026,6 +2096,10 @@
if (!old_dir->i_op || !old_dir->i_op->rename)
return -EPERM;

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

@@ -2118,7 +2118,7 @@
return error;
}

-static inline int do_rename(const char * oldname, const char * newname)
+int do_rename(const char * oldname, const char * newname)
{
int error = 0;
struct dentry * old_dir, * new_dir;
--- linux-2.6.5cow/include/linux/ext2_fs.h~madcow 2004-04-27 16:34:42.000000000 +0200
+++ linux-2.6.5cow/include/linux/ext2_fs.h 2004-04-27 17:01:30.000000000 +0200
@@ -192,6 +192,7 @@
#define EXT2_NOTAIL_FL 0x00008000 /* file tail should not be merged */
#define EXT2_DIRSYNC_FL 0x00010000 /* dirsync behaviour (directories only) */
#define EXT2_TOPDIR_FL 0x00020000 /* Top of directory hierarchies*/
+#define EXT2_MADCOWLINK_FL 0x00040000 /* COW behaviour for hard links */
#define EXT2_RESERVED_FL 0x80000000 /* reserved for ext2 lib */

#define EXT2_FL_USER_VISIBLE 0x0003DFFF /* User visible flags */
--- linux-2.6.5cow/include/linux/ext3_fs.h~madcow 2004-04-27 16:34:42.000000000 +0200
+++ linux-2.6.5cow/include/linux/ext3_fs.h 2004-04-27 17:03:46.000000000 +0200
@@ -185,6 +185,7 @@
#define EXT3_NOTAIL_FL 0x00008000 /* file tail should not be merged */
#define EXT3_DIRSYNC_FL 0x00010000 /* dirsync behaviour (directories only) */
#define EXT3_TOPDIR_FL 0x00020000 /* Top of directory hierarchies*/
+#define EXT3_MADCOWLINK_FL 0x00040000 /* COW behaviour for hard links */
#define EXT3_RESERVED_FL 0x80000000 /* reserved for ext3 lib */

#define EXT3_FL_USER_VISIBLE 0x0003DFFF /* User visible flags */
--- linux-2.6.5cow/include/linux/fcntl.h~madcow 2004-04-27 16:34:42.000000000 +0200
+++ linux-2.6.5cow/include/linux/fcntl.h 2004-04-27 16:49:45.000000000 +0200
@@ -23,6 +23,9 @@
#define DN_ATTRIB 0x00000020 /* File changed attibutes */
#define DN_MULTISHOT 0x80000000 /* Don't remove notifier */

+#define F_SETMADCOW (F_LINUX_SPECIFIC_BASE+3)
+#define F_GETMADCOW (F_LINUX_SPECIFIC_BASE+4)
+
#ifdef __KERNEL__

#if BITS_PER_LONG == 32
--- linux-2.6.5cow/include/linux/fs.h~madcow 2004-04-27 16:48:55.000000000 +0200
+++ linux-2.6.5cow/include/linux/fs.h 2004-04-30 08:40:04.000000000 +0200
@@ -90,6 +90,7 @@
/* public flags for file_system_type */
#define FS_REQUIRES_DEV 1
#define FS_BINARY_MOUNTDATA 2
+#define FS_MADCOW 4
#define FS_REVAL_DOT 16384 /* Check the paths ".", ".." for staleness */
#define FS_ODD_RENAME 32768 /* Temporary stuff; will go away as soon
* as nfs_rename() will be cleaned up
@@ -139,6 +140,9 @@
#define S_NOQUOTA 64 /* Inode is not counted to quota */
#define S_DIRSYNC 128 /* Directory modifications are synchronous */
#define S_NOCMTIME 256 /* Do not update file c/mtime */
+#define S_MADCOWLINK 512 /* 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
-
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