Re: [PATCH v2] inotify: Extend ioctl to allow to request id of new watch descriptor

On Sat 10-02-18 01:45:16, Kirill Tkhai wrote:

On 09.02.2018 23:56, Andrew Morton wrote:

quoted

On Fri, 9 Feb 2018 18:04:54 +0300 Kirill Tkhai [off-list ref] wrote:

quoted

Watch descriptor is id of the watch created by inotify_add_watch().
It is allocated in inotify_add_to_idr(), and takes the numbers
starting from 1. Every new inotify watch obtains next available
number (usually, old + 1), as served by idr_alloc_cyclic().

CRIU (Checkpoint/Restore In Userspace) project supports inotify
files, and restores watched descriptors with the same numbers,
they had before dump. Since there was no kernel support, we
had to use cycle to add a watch with specific descriptor id:

	while (1) {
		int wd;

		wd = inotify_add_watch(inotify_fd, path, mask);
		if (wd < 0) {
			break;
		} else if (wd == desired_wd_id) {
			ret = 0;
			break;
		}

		inotify_rm_watch(inotify_fd, wd);
	}

(You may find the actual code at the below link:
 https://github.com/checkpoint-restore/criu/blob/v3.7/criu/fsnotify.c#L577)

The cycle is suboptiomal and very expensive, but since there is no better
kernel support, it was the only way to restore that. Happily, we had met
mostly descriptors with small id, and this approach had worked somehow.

But recent time containers with inotify with big watch descriptors
begun to come, and this way stopped to work at all. When descriptor id
is something about 0x34d71d6, the restoring process spins in busy loop
for a long time, and the restore hungs and delay of migration from node
to node could easily be watched.

This patch aims to solve this problem. It introduces new ioctl
INOTIFY_IOC_SETNEXTWD, which allows to request the number of next created
watch descriptor from userspace. It simply calls idr_set_cursor() primitive
to populate idr::idr_next, so that next idr_alloc_cyclic() allocation
will return this id, if it is not occupied. This is the way which is
used to restore some other resources from userspace. For example,
/proc/sys/kernel/ns_last_pid works the same for task pids.

The new code is under CONFIG_CHECKPOINT_RESTORE #define, so small system
may exclude it.

Reviewed-by: Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>

With a little cleanup:

--- a/fs/notify/inotify/inotify_user.c~inotify-extend-ioctl-to-allow-to-request-id-of-new-watch-descriptor-fix
+++ a/fs/notify/inotify/inotify_user.c

@@ -285,7 +285,6 @@ static int inotify_release(struct inode
 static long inotify_ioctl(struct file *file, unsigned int cmd,
 			  unsigned long arg)
 {
-	struct inotify_group_private_data *data __maybe_unused;
 	struct fsnotify_group *group;
 	struct fsnotify_event *fsn_event;
 	void __user *p;

@@ -294,7 +293,6 @@ static long inotify_ioctl(struct file *f
 
 	group = file->private_data;
 	p = (void __user *) arg;
-	data = &group->inotify_data;
 
 	pr_debug("%s: group=%p cmd=%u\n", __func__, group, cmd);
 

@@ -313,6 +311,9 @@ static long inotify_ioctl(struct file *f
 	case INOTIFY_IOC_SETNEXTWD:
 		ret = -EINVAL;
 		if (arg >= 1 && arg <= INT_MAX) {
+			struct inotify_group_private_data *data;
+
+			data = &group->inotify_data;
 			spin_lock(&data->idr_lock);
 			idr_set_cursor(&data->idr, (unsigned int)arg);
 			spin_unlock(&data->idr_lock);

I have no objections.

Thanks guys, I have added the patch (with cleanup included) to my tree.

								Honza
-- 
Jan Kara [off-list ref]
SUSE Labs, CR