Re: [PATCH for testing] cow behaviour for hard links

From: Sytse Wielinga
Date: Fri Mar 12 2004 - 12:51:01 EST

Next message: Bartlomiej Zolnierkiewicz: "Re: kernelparameter idex=noprobe"
Previous message: Brad Laue: "Re: ksoftirqd using mysteriously high amounts of CPU time"
In reply to: Sytse Wielinga: "Re: [PATCH for testing] cow behaviour for hard links"
Next in thread: Jörn Engel: "Re: [PATCH for testing] cow behaviour for hard links"
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]

Hi,

I'm sorry to say this, but I stumbled upon a prohibitive problem...

The problem is that if a hard link would be broken up, one of the dentry's
would get a link to a new inode with a new inode number. This would mean that
right under the nose of the app, the file suddenly gets a new inode number.
Apps don't like that. If anyone has any suggestion that might make this
possible please say so, but I don't see it.

I have made some pretty thorough changes to your patch though. You can find
the patch attached to this email.
Things I've changed:

- moved break_cow_link from dentry_open in open.c to get_write_access in
namei.c. Putting it in dentry_open thoroughly breaks things, as it's too late
to save files from being truncated, for example.
- made something from the mess you made of ext2/ext3 inode flags :-P
- removed inheritance, as it's not useful in any way, not expected and breaks
linking of files with S_COWLINK set.
- made a go at supporting reiserfs, but failed... my changes are in the
patch, could somebody please have a look and tell me what I've missed?
- fcntl_setcow now spins a spinlock

Sytse
diff -rU 3 linux-2.6.4/fs/ext2/inode.c linux-2.6.4~cowlink/fs/ext2/inode.c
--- linux-2.6.4/fs/ext2/inode.c 2004-03-11 03:55:22.000000000 +0100
+++ linux-2.6.4~cowlink/fs/ext2/inode.c 2004-03-12 11:56:56.000000000 +0100
@@ -1135,6 +1135,9 @@
}
brelse (bh);
ext2_set_inode_flags(inode);
+ inode->i_flags &= ~S_COWLINK;
+ if (ei->i_flags & EXT2_COWLINK_FL)
+ inode->i_flags |= S_COWLINK;
return;

bad_inode:
@@ -1151,9 +1154,13 @@
gid_t gid = inode->i_gid;
struct buffer_head * bh;
struct ext2_inode * raw_inode = ext2_get_inode(sb, ino, &bh);
+ unsigned int flags = ei->i_flags & ~EXT2_COWLINK_FL;
int n;
int err = 0;

+ if (inode->i_flags & S_COWLINK)
+ flags |= EXT2_COWLINK_FL;
+
if (IS_ERR(raw_inode))
return -EIO;

@@ -1191,7 +1198,7 @@

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(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;
diff -rU 3 linux-2.6.4/fs/ext3/inode.c linux-2.6.4~cowlink/fs/ext3/inode.c
--- linux-2.6.4/fs/ext3/inode.c 2004-03-11 03:55:35.000000000 +0100
+++ linux-2.6.4~cowlink/fs/ext3/inode.c 2004-03-12 11:56:56.000000000 +0100
@@ -2569,6 +2569,9 @@
}
brelse (iloc.bh);
ext3_set_inode_flags(inode);
+ inode->i_flags &= ~S_COWLINK;
+ if (ei->i_flags & EXT3_COWLINK_FL)
+ inode->i_flags |= S_COWLINK;
return;

bad_inode:
@@ -2590,8 +2593,12 @@
struct ext3_inode *raw_inode = ext3_raw_inode(iloc);
struct ext3_inode_info *ei = EXT3_I(inode);
struct buffer_head *bh = iloc->bh;
+ unsigned int flags = ei->i_flags & ~EXT3_COWLINK_FL;
int err = 0, rc, block;

+ if (inode->i_flags & S_COWLINK)
+ flags |= EXT3_COWLINK_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)
@@ -2629,7 +2636,7 @@
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(flags);
#ifdef EXT3_FRAGMENTS
raw_inode->i_faddr = cpu_to_le32(ei->i_faddr);
raw_inode->i_frag = ei->i_frag_no;
diff -rU 3 linux-2.6.4/fs/fcntl.c linux-2.6.4~cowlink/fs/fcntl.c
--- linux-2.6.4/fs/fcntl.c 2004-03-11 03:55:35.000000000 +0100
+++ linux-2.6.4~cowlink/fs/fcntl.c 2004-03-12 13:41:44.000000000 +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;
}
diff -rU 3 linux-2.6.4/fs/namei.c linux-2.6.4~cowlink/fs/namei.c
--- linux-2.6.4/fs/namei.c 2004-03-11 03:55:25.000000000 +0100
+++ linux-2.6.4~cowlink/fs/namei.c 2004-03-12 13:11:03.000000000 +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 -EPERM;
+}
+
+/*
* get_write_access() gets write permission for a file.
* put_write_access() releases this write permission.
* This is used for regular files.
@@ -243,7 +270,15 @@

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;
@@ -1522,6 +1557,7 @@
if (!error) {
inode_dir_notify(dir, DN_CREATE);
security_inode_post_mkdir(dir,dentry, mode);
+ dentry->d_inode->i_flags |= dir->i_flags & S_COWLINK;
}
return error;
}
diff -rU 3 linux-2.6.4/fs/reiserfs/inode.c linux-2.6.4~cowlink/fs/reiserfs/inode.c
--- linux-2.6.4/fs/reiserfs/inode.c 2004-03-11 03:55:25.000000000 +0100
+++ linux-2.6.4~cowlink/fs/reiserfs/inode.c 2004-03-12 11:56:56.000000000 +0100
@@ -2303,6 +2303,10 @@
inode -> i_flags |= S_NOATIME;
else
inode -> i_flags &= ~S_NOATIME;
+ if( sd_attrs & REISERFS_COWLINK_FL )
+ inode -> i_flags |= S_COWLINK;
+ else
+ inode -> i_flags &= ~S_COWLINK;
if( sd_attrs & REISERFS_NOTAIL_FL )
REISERFS_I(inode)->i_flags |= i_nopack_mask;
else
@@ -2325,6 +2329,10 @@
*sd_attrs |= REISERFS_NOATIME_FL;
else
*sd_attrs &= ~REISERFS_NOATIME_FL;
+ if( inode -> i_flags & S_COWLINK )
+ *sd_attrs |= REISERFS_COWLINK_FL;
+ else
+ *sd_attrs &= ~REISERFS_COWLINK_FL;
if( REISERFS_I(inode)->i_flags & i_nopack_mask )
*sd_attrs |= REISERFS_NOTAIL_FL;
else
diff -rU 3 linux-2.6.4/include/linux/ext2_fs.h linux-2.6.4~cowlink/include/linux/ext2_fs.h
--- linux-2.6.4/include/linux/ext2_fs.h 2004-03-11 03:55:22.000000000 +0100
+++ linux-2.6.4~cowlink/include/linux/ext2_fs.h 2004-03-12 11:56:56.000000000 +0100
@@ -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_COWLINK_FL 0x00040000 /* Copy On Write */
#define EXT2_RESERVED_FL 0x80000000 /* reserved for ext2 lib */

#define EXT2_FL_USER_VISIBLE 0x0003DFFF /* User visible flags */
diff -rU 3 linux-2.6.4/include/linux/ext3_fs.h linux-2.6.4~cowlink/include/linux/ext3_fs.h
--- linux-2.6.4/include/linux/ext3_fs.h 2004-03-11 03:55:33.000000000 +0100
+++ linux-2.6.4~cowlink/include/linux/ext3_fs.h 2004-03-12 11:56:56.000000000 +0100
@@ -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_COWLINK_FL 0x00040000 /* Copy On Write */
#define EXT3_RESERVED_FL 0x80000000 /* reserved for ext3 lib */

#define EXT3_FL_USER_VISIBLE 0x0003DFFF /* User visible flags */
diff -rU 3 linux-2.6.4/include/linux/fcntl.h linux-2.6.4~cowlink/include/linux/fcntl.h
--- linux-2.6.4/include/linux/fcntl.h 2004-03-11 03:55:24.000000000 +0100
+++ linux-2.6.4~cowlink/include/linux/fcntl.h 2004-03-12 11:56:56.000000000 +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
diff -rU 3 linux-2.6.4/include/linux/fs.h linux-2.6.4~cowlink/include/linux/fs.h
--- linux-2.6.4/include/linux/fs.h 2004-03-11 03:55:24.000000000 +0100
+++ linux-2.6.4~cowlink/include/linux/fs.h 2004-03-12 13:34:49.000000000 +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 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
@@ -171,6 +174,8 @@

#define IS_DEADDIR(inode) ((inode)->i_flags & S_DEAD)

+#define IS_COWLINK(inode) ((inode)->i_flags & S_COWLINK)
+
/* the read-only stuff doesn't really belong here, but any other place is
probably as bad and I don't want to create yet another include file. */

diff -rU 3 linux-2.6.4/include/linux/reiserfs_fs.h linux-2.6.4~cowlink/include/linux/reiserfs_fs.h
--- linux-2.6.4/include/linux/reiserfs_fs.h 2004-03-11 03:55:44.000000000 +0100
+++ linux-2.6.4~cowlink/include/linux/reiserfs_fs.h 2004-03-12 11:56:56.000000000 +0100
@@ -887,6 +887,12 @@
#define REISERFS_UNRM_FL EXT2_UNRM_FL
#define REISERFS_COMPR_FL EXT2_COMPR_FL
#define REISERFS_NOTAIL_FL EXT2_NOTAIL_FL
+/* TODO: Check how the inheriting of flags work, add this to the inherit mask,
+ * make this a special flag for ext2 too, instead of a special ioctl/flag, and
+ * add it to chattr/lsattr.
+ */
+/* This one is never going to be used in reiserfs 3... I hope */
+#define REISERFS_COWLINK_FL EXT2_BTREE_FL /* Copy On Write */

/* persistent flags that file inherits from the parent directory */
#define REISERFS_INHERIT_MASK ( REISERFS_IMMUTABLE_FL | \

Next message: Bartlomiej Zolnierkiewicz: "Re: kernelparameter idex=noprobe"
Previous message: Brad Laue: "Re: ksoftirqd using mysteriously high amounts of CPU time"
In reply to: Sytse Wielinga: "Re: [PATCH for testing] cow behaviour for hard links"
Next in thread: Jörn Engel: "Re: [PATCH for testing] cow behaviour for hard links"
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]