Re: [PATCH] fsnotify: Extend ioctl to allow to suppress events

Sheng Yong <shengyong2021@xxxxxxxxx> · Fri, 17 Sep 2021 20:05:28 +0800

Hi, Honza,

On 2021/9/17 16:49, Jan Kara wrote:
On Thu 16-09-21 22:06:49, Sheng Yong wrote:
This patch adds a new ioctl cmd INOTIFY_IOC_SUPRESS to stop queueing
events temporarily. A second ioctl could resume queueing events again.
With it enabled, we could easily suppress events instead of updating
or removing watches, especially if we have losts of watches referring
to one inotify instance, for example watching a directory recursively.

Signed-off-by: Sheng Yong <shengyong2021@xxxxxxxxx>

Thanks for the patch! This ioctl on its own is equivalent to shutting down
the notification group so I don't think it is really useful on its own. 

Thanks for your reply.
Yes, this ioctl reuses the stop-queueing-events procedure of shutdown.

If you add ioctl to resume queueing, it makes some sense but can you
ellaborate a bit more why you need to stop receiving events temporarily?

The usecase is that we use rsync+inotify to backup files. Let's say there are
two tasks: 1) "real-time" task reads events and syncs the file; 2) periodic
task syncs all files recursively in the source directory.

If the queue get overflowed and new events are discarded, we have to start a
recursive sync. But recursive sync takes a very long time to scan, compare and
transmit data, we prefer a periodic task to do that, so that we could ignore
overflow events.

When the periodic task is working, I think the "real-time" task should stop
reading and handling new events temporarily to avoid duplicated IO in both
sides. I think stop queueing new events is the easiest way to do that.

thanks,
Sheng

You'll have to rescan whole directory hierarchy anyway after resuming to be
able to see what has changed so the usefullness of this feature is unclear
to me...

Also we generally don't add new functionality to inotify and rather direct
that towards fanotify and finally I'm not sure ioctl() is the best API for
this but let's cover these questions once the usecase is clear.

								Honza

---
  fs/notify/fanotify/fanotify_user.c |  2 +-
  fs/notify/group.c                  | 12 +++++++++---
  fs/notify/inotify/inotify_user.c   |  4 ++++
  fs/notify/mark.c                   |  3 ++-
  fs/notify/notification.c           |  4 ++--
  include/linux/fsnotify_backend.h   | 10 +++++++---
  include/uapi/linux/inotify.h       |  3 +++
  7 files changed, 28 insertions(+), 10 deletions(-)

diff --git a/fs/notify/fanotify/fanotify_user.c b/fs/notify/fanotify/fanotify_user.c
index 6facdf476255..1e24738762d3 100644
--- a/fs/notify/fanotify/fanotify_user.c
+++ b/fs/notify/fanotify/fanotify_user.c
@@ -773,7 +773,7 @@ static int fanotify_release(struct inode *ignored, struct file *file)
  	 * userspace cannot use fanotify fd anymore, no event can enter or
  	 * leave access_list by now either.
  	 */
-	fsnotify_group_stop_queueing(group);
+	fsnotify_group_stop_queueing(group, FS_GRP_SHUTDOWN);
  
  	/*
  	 * Process all permission events on access_list and notification queue
diff --git a/fs/notify/group.c b/fs/notify/group.c
index fb89c351295d..ce62ce6caf30 100644
--- a/fs/notify/group.c
+++ b/fs/notify/group.c
@@ -34,10 +34,16 @@ static void fsnotify_final_destroy_group(struct fsnotify_group *group)
   * Stop queueing new events for this group. Once this function returns
   * fsnotify_add_event() will not add any new events to the group's queue.
   */
-void fsnotify_group_stop_queueing(struct fsnotify_group *group)
+void fsnotify_group_stop_queueing(struct fsnotify_group *group, unsigned int st)
  {
+	if (st & ~FS_GRP_STOP_QUEUEING)
+		return;
+
  	spin_lock(&group->notification_lock);
-	group->shutdown = true;
+	if (group->state & st)
+		group->state &= ~st;
+	else
+		group->state |= st;
  	spin_unlock(&group->notification_lock);
  }
  
@@ -55,7 +61,7 @@ void fsnotify_destroy_group(struct fsnotify_group *group)
  	 * fsnotify_destroy_group() is called and this makes the other callers
  	 * of fsnotify_destroy_group() to see the same behavior.
  	 */
-	fsnotify_group_stop_queueing(group);
+	fsnotify_group_stop_queueing(group, FS_GRP_SHUTDOWN);
  
  	/* Clear all marks for this group and queue them for destruction */
  	fsnotify_clear_marks_by_group(group, FSNOTIFY_OBJ_ALL_TYPES_MASK);
diff --git a/fs/notify/inotify/inotify_user.c b/fs/notify/inotify/inotify_user.c
index 62051247f6d2..67cf47f1943b 100644
--- a/fs/notify/inotify/inotify_user.c
+++ b/fs/notify/inotify/inotify_user.c
@@ -327,6 +327,10 @@ static long inotify_ioctl(struct file *file, unsigned int cmd,
  		}
  		break;
  #endif /* CONFIG_CHECKPOINT_RESTORE */
+	case INOTIFY_IOC_SUPPRESS:
+		fsnotify_group_stop_queueing(group, FS_GRP_SUPPRESS);
+		ret = 0;
+		break;
  	}
  
  	return ret;
diff --git a/fs/notify/mark.c b/fs/notify/mark.c
index fa1d99101f89..08f9d1e480de 100644
--- a/fs/notify/mark.c
+++ b/fs/notify/mark.c
@@ -343,7 +343,8 @@ static void fsnotify_put_mark_wake(struct fsnotify_mark *mark)
  		 * We abuse notification_waitq on group shutdown for waiting for
  		 * all marks pinned when waiting for userspace.
  		 */
-		if (atomic_dec_and_test(&group->user_waits) && group->shutdown)
+		if (atomic_dec_and_test(&group->user_waits) &&
+		    group->state & FS_GRP_SHUTDOWN)
  			wake_up(&group->notification_waitq);
  	}
  }
diff --git a/fs/notify/notification.c b/fs/notify/notification.c
index 32f45543b9c6..6586e09e9141 100644
--- a/fs/notify/notification.c
+++ b/fs/notify/notification.c
@@ -76,7 +76,7 @@ void fsnotify_destroy_event(struct fsnotify_group *group,
   * 0 if the event was added to a queue
   * 1 if the event was merged with some other queued event
   * 2 if the event was not queued - either the queue of events has overflown
- *   or the group is shutting down.
+ *   or the group is suppressing or shutting down.
   */
  int fsnotify_add_event(struct fsnotify_group *group,
  		       struct fsnotify_event *event,
@@ -92,7 +92,7 @@ int fsnotify_add_event(struct fsnotify_group *group,
  
  	spin_lock(&group->notification_lock);
  
-	if (group->shutdown) {
+	if (group->state & FS_GRP_STOP_QUEUEING) {
  		spin_unlock(&group->notification_lock);
  		return 2;
  	}
diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h
index 1ce66748a2d2..1f9b2afb26cb 100644
--- a/include/linux/fsnotify_backend.h
+++ b/include/linux/fsnotify_backend.h
@@ -103,6 +103,10 @@
  
  #define ALL_FSNOTIFY_BITS   (ALL_FSNOTIFY_EVENTS | ALL_FSNOTIFY_FLAGS)
  
+#define FS_GRP_SHUTDOWN 0x1	/* group is being shut down, don't queue more events */
+#define FS_GRP_SUPPRESS 0x2	/* group is being suppressed, don't queue more events */
+#define FS_GRP_STOP_QUEUEING (FS_GRP_SHUTDOWN | FS_GRP_SUPPRESS)
+
  struct fsnotify_group;
  struct fsnotify_event;
  struct fsnotify_mark;
@@ -202,7 +206,7 @@ struct fsnotify_group {
  	#define FS_PRIO_1	1 /* fanotify content based access control */
  	#define FS_PRIO_2	2 /* fanotify pre-content access */
  	unsigned int priority;
-	bool shutdown;		/* group is being shut down, don't queue more events */
+	unsigned int state;
  
  	/* stores all fastpath marks assoc with this group so they can be cleaned on unregister */
  	struct mutex mark_mutex;	/* protect marks_list */
@@ -472,8 +476,8 @@ extern struct fsnotify_group *fsnotify_alloc_user_group(const struct fsnotify_op
  extern void fsnotify_get_group(struct fsnotify_group *group);
  /* drop reference on a group from fsnotify_alloc_group */
  extern void fsnotify_put_group(struct fsnotify_group *group);
-/* group destruction begins, stop queuing new events */
-extern void fsnotify_group_stop_queueing(struct fsnotify_group *group);
+/* group destruction begins or suppresses, stop queuing new events */
+extern void fsnotify_group_stop_queueing(struct fsnotify_group *group, unsigned int st);
  /* destroy group */
  extern void fsnotify_destroy_group(struct fsnotify_group *group);
  /* fasync handler function */
diff --git a/include/uapi/linux/inotify.h b/include/uapi/linux/inotify.h
index 884b4846b630..07155241d5a9 100644
--- a/include/uapi/linux/inotify.h
+++ b/include/uapi/linux/inotify.h
@@ -78,7 +78,10 @@ struct inotify_event {
   *
   * INOTIFY_IOC_SETNEXTWD: set desired number of next created
   * watch descriptor.
+ *
+ * INOTIFY_IOC_SUPPRESS: suppress events temporarily.
   */
  #define INOTIFY_IOC_SETNEXTWD	_IOW('I', 0, __s32)
+#define INOTIFY_IOC_SUPPRESS	_IOW('I', 1, __s32)
  
  #endif /* _UAPI_LINUX_INOTIFY_H */
--
2.20.1







