Re: [RFC PATCH] Filesystem Journal Notifications

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Thanks for your feedback.
It gave me new points to think about.

I have used inotify simply because it is a mechanism that already exists
in the kernel and is easily extensible.

The use of transaction ids as "journal cookie" was done to simplify
things. A journal transaction ID is a readily available monotonically
increasing ID unique to that partition (super block).

Journal cookies themselves are completely opaque to the application
and as long as it is a monotonically increasing number that signifies
a journal commit, it could be anything that a file system chooses it
to be.

Ext3/Ext4 gather multiple changes into a single transaction. So,
you will see a single 'commit' notification for multiple of meta-data
changes. The 'commit' messages, therefore, won't be that many.


This patch (v2):

In this version of the patch I've done away with most of the file-system
specific changes. The file system now only needs to send out a journal
commit notification.

The 'journal cookie' is simply a u64 that increases every time a journal
'commit' is made. This will make the code more portable to other file
systems.

Also, we no longer need those many journal message types. We can easily
piggy-back on top of fsnotify. The only remaining one is
IN_JOURNAL_COMMIT.

My next step is to allow multiple registrations for JOURNAL_COMMIT
notification. I will post that patch once it is ready.

Signed-off-by: Abhijit Paithankar <apaithan@xxxxxxxxxx>
---

 fs/ext3/ext3_jbd.c       |    9 +++++
 fs/ext4/ext4_jbd2.c      |    9 +++++
 fs/inotify.c             |   17 ++++++---
 fs/inotify_user.c        |   81 ++++++++++++++++++++++++++++++++++++++++++-----
 fs/jbd/commit.c          |    1 
 fs/jbd2/commit.c         |    1 
 include/linux/fs.h       |    2 +
 include/linux/fsnotify.h |   11 ++++++
 include/linux/inotify.h  |   19 ++++++++++-
 include/linux/jbd.h      |    1 
 include/linux/jbd2.h     |    1 
 11 files changed, 138 insertions(+), 14 deletions(-)

---


Index: linux-2.6/fs/ext3/ext3_jbd.c
===================================================================
--- linux-2.6.orig/fs/ext3/ext3_jbd.c	2008-09-15 10:12:25.000000000 -0700
+++ linux-2.6/fs/ext3/ext3_jbd.c	2008-09-15 10:15:54.000000000 -0700
@@ -3,6 +3,7 @@
  */
 
 #include <linux/ext3_jbd.h>
+#include <linux/fsnotify.h>
 
 int __ext3_journal_get_undo_access(const char *where, handle_t *handle,
 				struct buffer_head *bh)
@@ -57,3 +58,11 @@
 		ext3_journal_abort_handle(where, __func__, bh, handle,err);
 	return err;
 }
+
+void journal_notify_commit (journal_t *journal)
+{
+	struct super_block *sb = journal->j_private;
+	if (sb->s_priv_inode && sb->s_journal_cookie)
+		fsnotify_journal_commit(sb);
+}
+
Index: linux-2.6/fs/ext4/ext4_jbd2.c
===================================================================
--- linux-2.6.orig/fs/ext4/ext4_jbd2.c	2008-09-15 10:14:12.000000000 -0700
+++ linux-2.6/fs/ext4/ext4_jbd2.c	2008-09-15 10:15:50.000000000 -0700
@@ -2,6 +2,7 @@
  * Interface between ext4 and JBD
  */
 
+#include <linux/fsnotify.h>
 #include "ext4_jbd2.h"
 
 int __ext4_journal_get_undo_access(const char *where, handle_t *handle,
@@ -57,3 +58,11 @@
 		ext4_journal_abort_handle(where, __func__, bh, handle, err);
 	return err;
 }
+
+void jbd2_journal_notify_commit (journal_t *journal)
+{
+	struct super_block *sb = journal->j_private;
+	if (sb->s_priv_inode && sb->s_journal_cookie)
+		fsnotify_journal_commit(sb);
+}
+
Index: linux-2.6/fs/inotify.c
===================================================================
--- linux-2.6.orig/fs/inotify.c	2008-09-15 10:12:29.000000000 -0700
+++ linux-2.6/fs/inotify.c	2008-09-15 10:13:44.000000000 -0700
@@ -231,7 +231,7 @@
 				 struct inotify_watch *watch)
 {
 	remove_watch_no_event(watch, ih);
-	ih->in_ops->handle_event(watch, watch->wd, IN_IGNORED, 0, NULL, NULL);
+	ih->in_ops->handle_event(watch, watch->wd, IN_IGNORED, 0, NULL, NULL, 0);
 }
 EXPORT_SYMBOL_GPL(inotify_remove_watch_locked);
 
@@ -280,10 +280,17 @@
 			       const char *name, struct inode *n_inode)
 {
 	struct inotify_watch *watch, *next;
+	u64 journal_cookie = 0;
 
 	if (!inotify_inode_watched(inode))
 		return;
 
+	if (mask & (IN_ATTRIB | IN_MOVE | IN_CREATE | IN_DELETE |
+		    IN_DELETE_SELF | IN_MOVE_SELF)) {
+		if (inode->i_sb && inode->i_sb->s_journal_cookie)
+			journal_cookie = inode->i_sb->s_journal_cookie;
+	}
+
 	mutex_lock(&inode->inotify_mutex);
 	list_for_each_entry_safe(watch, next, &inode->inotify_watches, i_list) {
 		u32 watch_mask = watch->mask;
@@ -293,7 +300,7 @@
 			if (watch_mask & IN_ONESHOT)
 				remove_watch_no_event(watch, ih);
 			ih->in_ops->handle_event(watch, watch->wd, mask, cookie,
-						 name, n_inode);
+						 name, n_inode, journal_cookie);
 			mutex_unlock(&ih->mutex);
 		}
 	}
@@ -409,7 +416,7 @@
 			struct inotify_handle *ih= watch->ih;
 			mutex_lock(&ih->mutex);
 			ih->in_ops->handle_event(watch, watch->wd, IN_UNMOUNT, 0,
-						 NULL, NULL);
+						 NULL, NULL, 0);
 			inotify_remove_watch_locked(ih, watch);
 			mutex_unlock(&ih->mutex);
 		}
@@ -576,7 +583,7 @@
 		mask_add = 1;
 
 	/* don't allow invalid bits: we don't want flags set */
-	mask &= IN_ALL_EVENTS | IN_ONESHOT;
+	mask &= IN_ALL_EVENTS | IN_ONESHOT | IN_JOURNAL_COMMIT;
 	if (unlikely(!mask))
 		return -EINVAL;
 
@@ -623,7 +630,7 @@
 	int newly_watched;
 
 	/* don't allow invalid bits: we don't want flags set */
-	mask &= IN_ALL_EVENTS | IN_ONESHOT;
+	mask &= IN_ALL_EVENTS | IN_ONESHOT | IN_JOURNAL_COMMIT;
 	if (unlikely(!mask))
 		return -EINVAL;
 	watch->mask = mask;
Index: linux-2.6/fs/inotify_user.c
===================================================================
--- linux-2.6.orig/fs/inotify_user.c	2008-09-15 10:12:34.000000000 -0700
+++ linux-2.6/fs/inotify_user.c	2008-09-15 10:51:43.000000000 -0700
@@ -185,7 +185,7 @@
  * This function can sleep.
  */
 static struct inotify_kernel_event * kernel_event(s32 wd, u32 mask, u32 cookie,
-						  const char *name)
+						  const char *name, u64 journal_cookie)
 {
 	struct inotify_kernel_event *kevent;
 
@@ -204,6 +204,7 @@
 
 	if (name) {
 		size_t len, rem, event_size = sizeof(struct inotify_event);
+		size_t journal_cookie_sz;
 
 		/*
 		 * We need to pad the filename so as to properly align an
@@ -213,6 +214,10 @@
 		 * simple and safe for all architectures.
 		 */
 		len = strlen(name) + 1;
+		if (journal_cookie) {
+			journal_cookie_sz = sizeof(u64);
+			len = len + journal_cookie_sz;
+		}
 		rem = event_size - len;
 		if (len > event_size) {
 			rem = event_size - (len % event_size);
@@ -225,13 +230,52 @@
 			kmem_cache_free(event_cachep, kevent);
 			return NULL;
 		}
-		memcpy(kevent->name, name, len);
+		if (journal_cookie) {
+			char *ch;
+
+			memcpy(kevent->name, name, strlen(name));
+			ch = kevent->name;
+			ch += strlen(name);
+			memset(ch++, 0, 1);
+			sprintf(ch, "%llu", journal_cookie);
+		}
+		else {
+			memcpy(kevent->name, name, len);
+		}
 		if (rem)
 			memset(kevent->name + len, 0, rem);
 		kevent->event.len = len + rem;
 	} else {
-		kevent->event.len = 0;
-		kevent->name = NULL;
+		if (journal_cookie) {
+			size_t len, rem, event_size = sizeof(struct inotify_event);
+			size_t journal_cookie_sz = sizeof(u64);
+			char *ch;
+
+			len = 1 + journal_cookie_sz;
+			rem = event_size - len;
+			if (len > event_size) {
+				rem = event_size - (len % event_size);
+				if (len % event_size == 0)
+					rem = 0;
+			}
+
+			kevent->name = kmalloc(len + rem, GFP_KERNEL);
+			if (unlikely(!kevent->name)) {
+				kmem_cache_free(event_cachep, kevent);
+				return NULL;
+			}
+			memset(kevent->name, 0, 1);
+			ch = kevent->name;
+			ch += 1;
+			sprintf(ch, "%llu", journal_cookie);
+			if (rem)
+				memset(kevent->name + len, 0, rem);
+			kevent->event.len = len + rem;
+		}
+		else {
+			kevent->event.len = 0;
+			kevent->name = NULL;
+		}
 	}
 
 	return kevent;
@@ -269,7 +313,7 @@
  */
 static void inotify_dev_queue_event(struct inotify_watch *w, u32 wd, u32 mask,
 				    u32 cookie, const char *name,
-				    struct inode *ignored)
+				    struct inode *ignored, u64 journal_cookie)
 {
 	struct inotify_user_watch *watch;
 	struct inotify_device *dev;
@@ -280,11 +324,25 @@
 
 	mutex_lock(&dev->ev_mutex);
 
+	if (mask & IN_JOURNAL_COMMIT) {
+		if (!(w->inode) || !(w->inode->i_sb)
+			|| !(w->inode->i_sb->s_priv_inode)
+			|| !(w->inode->i_sb->s_journal_cookie))
+			goto out;
+	}
 	/* we can safely put the watch as we don't reference it while
 	 * generating the event
 	 */
-	if (mask & IN_IGNORED || w->mask & IN_ONESHOT)
+	if (mask & IN_IGNORED || w->mask & IN_ONESHOT) {
+                if (w->mask & IN_JOURNAL_COMMIT) {
+                        struct super_block *sb = NULL;
+                        if (w->inode && w->inode->i_sb)
+                                sb = w->inode->i_sb;
+                        if (sb && sb->s_priv_inode)
+                                sb->s_priv_inode = NULL;
+                }
 		put_inotify_watch(w); /* final put */
+	}
 
 	/* coalescing: drop this event if it is a dupe of the previous */
 	last = inotify_dev_get_last_event(dev);
@@ -304,9 +362,9 @@
 
 	/* if the queue overflows, we need to notify user space */
 	if (unlikely(dev->event_count == dev->max_events))
-		kevent = kernel_event(-1, IN_Q_OVERFLOW, cookie, NULL);
+		kevent = kernel_event(-1, IN_Q_OVERFLOW, cookie, NULL, 0);
 	else
-		kevent = kernel_event(wd, mask, cookie, name);
+		kevent = kernel_event(wd, mask, cookie, name, journal_cookie);
 
 	if (unlikely(!kevent))
 		goto out;
@@ -686,6 +744,13 @@
 	ret = inotify_find_update_watch(dev->ih, inode, mask);
 	if (ret == -ENOENT)
 		ret = create_watch(dev, inode, mask);
+	if (ret >= 0) {
+		if (mask & IN_JOURNAL_COMMIT) {
+                        struct super_block *sb = inode->i_sb;
+                        if (sb)
+                                sb->s_priv_inode = inode;
+		}
+	}
 	mutex_unlock(&dev->up_mutex);
 
 	path_put(&path);
Index: linux-2.6/fs/jbd/commit.c
===================================================================
--- linux-2.6.orig/fs/jbd/commit.c	2008-09-15 10:12:42.000000000 -0700
+++ linux-2.6/fs/jbd/commit.c	2008-09-15 10:13:44.000000000 -0700
@@ -928,6 +928,7 @@
 	}
 	spin_unlock(&journal->j_list_lock);
 
+	journal_notify_commit(journal);
 	jbd_debug(1, "JBD: commit %d complete, head %d\n",
 		  journal->j_commit_sequence, journal->j_tail_sequence);
 
Index: linux-2.6/fs/jbd2/commit.c
===================================================================
--- linux-2.6.orig/fs/jbd2/commit.c	2008-09-15 10:14:00.000000000 -0700
+++ linux-2.6/fs/jbd2/commit.c	2008-09-15 10:16:09.000000000 -0700
@@ -990,6 +990,7 @@
 	}
 	spin_unlock(&journal->j_list_lock);
 
+	jbd2_journal_notify_commit(journal);
 	jbd_debug(1, "JBD: commit %d complete, head %d\n",
 		  journal->j_commit_sequence, journal->j_tail_sequence);
 
Index: linux-2.6/include/linux/fs.h
===================================================================
--- linux-2.6.orig/include/linux/fs.h	2008-09-15 10:12:47.000000000 -0700
+++ linux-2.6/include/linux/fs.h	2008-09-15 10:13:44.000000000 -0700
@@ -1106,6 +1106,8 @@
 	char s_id[32];				/* Informational name */
 
 	void 			*s_fs_info;	/* Filesystem private info */
+	u64			s_journal_cookie;	/* current journal cookie # */
+	struct inode		*s_priv_inode;	/* used for journal notifications */
 
 	/*
 	 * The next field is for VFS *only*. No filesystems have any business
Index: linux-2.6/include/linux/fsnotify.h
===================================================================
--- linux-2.6.orig/include/linux/fsnotify.h	2008-09-15 10:12:53.000000000 -0700
+++ linux-2.6/include/linux/fsnotify.h	2008-09-15 10:13:44.000000000 -0700
@@ -34,6 +34,17 @@
 	inotify_d_move(entry);
 }
 
+static inline void fsnotify_journal_commit (struct super_block *sb)
+{
+	struct inode *inode;
+	sb->s_journal_cookie += 1;
+	if (sb->s_journal_cookie && sb->s_priv_inode) {
+		inode = sb->s_priv_inode;
+		inotify_inode_queue_event(inode, IN_JOURNAL_COMMIT, 0,
+					  NULL, NULL);
+	}
+}
+
 /*
  * fsnotify_move - file old_name at old_dir was moved to new_name at new_dir
  */
Index: linux-2.6/include/linux/inotify.h
===================================================================
--- linux-2.6.orig/include/linux/inotify.h	2008-09-15 10:12:59.000000000 -0700
+++ linux-2.6/include/linux/inotify.h	2008-09-15 10:13:44.000000000 -0700
@@ -44,6 +44,23 @@
 #define IN_Q_OVERFLOW		0x00004000	/* Event queued overflowed */
 #define IN_IGNORED		0x00008000	/* File was ignored */
 
+/* This is a journal commit event for file systems. These events are sent
+ * from a journaling filesystem or the journal itself.
+ *
+ * IN_JOURNAL_COMMIT is special in that only one file per journaling filesystem
+ * partition (super-block) is allowed. Registering for this event with multiple
+ * files will overwrite previous registrations. The last registration for this
+ * event will be taken as current and commit notifications will be delivered
+ * for that inode only.
+ *
+ * The inode registered for this event should not be unlinked. This will disable
+ * any further journal commit notifications.
+ *
+ * If this event is not registered for, journal commit messages will not be
+ * delivered
+ */
+#define IN_JOURNAL_COMMIT	0x00010000	/* Journal on this superblock has committed a transaction */
+
 /* helper events */
 #define IN_CLOSE		(IN_CLOSE_WRITE | IN_CLOSE_NOWRITE) /* close */
 #define IN_MOVE			(IN_MOVED_FROM | IN_MOVED_TO) /* moves */
@@ -97,7 +114,7 @@
 
 struct inotify_operations {
 	void (*handle_event)(struct inotify_watch *, u32, u32, u32,
-			     const char *, struct inode *);
+			     const char *, struct inode *, u64);
 	void (*destroy_watch)(struct inotify_watch *);
 };
 
Index: linux-2.6/include/linux/jbd.h
===================================================================
--- linux-2.6.orig/include/linux/jbd.h	2008-09-15 10:13:08.000000000 -0700
+++ linux-2.6/include/linux/jbd.h	2008-09-15 10:13:44.000000000 -0700
@@ -114,6 +114,7 @@
  * This is an opaque datatype.
  **/
 typedef struct journal_s	journal_t;	/* Journal control structure */
+extern void journal_notify_commit(journal_t *journal);
 #endif
 
 /*
Index: linux-2.6/include/linux/jbd2.h
===================================================================
--- linux-2.6.orig/include/linux/jbd2.h	2008-09-15 10:16:18.000000000 -0700
+++ linux-2.6/include/linux/jbd2.h	2008-09-15 10:16:45.000000000 -0700
@@ -115,6 +115,7 @@
  * This is an opaque datatype.
  **/
 typedef struct journal_s	journal_t;	/* Journal control structure */
+extern void jbd2_journal_notify_commit(journal_t *journal);
 #endif
 
 /*

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