Re: [PATCH 001 of 6] md: Send online/offline uevents when an md array starts/stops.

[Neil Brown <neilb@suse.de>]

On Monday November 6, dean@arctic.org wrote:
> On Mon, 6 Nov 2006, Neil Brown wrote:
> 
> > This creates a deep disconnect between udev and md.
> > udev expects a device to appear first, then it created the
> > device-special-file in /dev.
> > md expect the device-special-file to exist first, and then created the
> > device on the first open.
> 
> could you create a special /dev/mdx device which is used to 
> assemble/create arrays only?  i mean literally "mdx" not "mdX" where X is 
> a number.  mdx would always be there if md module is loaded... so udev 
> would see the driver appear and then create the /dev/mdx.  then mdadm 
> would use /dev/mdx to do assemble/creates/whatever and cause other devices 
> to appear/disappear in a manner which udev is happy with.
> 
> (much like how /dev/ptmx is used to create /dev/pts/N entries.)
> 
> doesn't help legacy mdadm binaries... but seems like it fits the New World 
> Order.
> 
> or hm i suppose the New World Order is to eschew binary interfaces and 
> suggest a /sys/class/md/ hierarchy with a bunch of files you have to splat 
> ascii data into to cause an array to be created/assembled.

I have the following patch sitting in my patch queue (since about
March).
It does what you suggest via /sys/module/md-mod/parameters/MAGIC_FILE
which is the only md-specific part of the /sys namespace that I could
find.

However I'm not at all convinced that it is a good idea.  I would much
rather have mdadm control device naming than leave it up to udev.

An in any case, we have the semantic that opening an md device-file
creates the device, and we cannot get rid of that semantic without a
lot of warning and a lot of pain.  And adding a new semantic isn't
really going to help.

We simply need to find the best way for udev and md to play together,
and I think we can achieve something quite workable.  Both sides just
have to give a bit.

NeilBrown


Allow md devices to be created by writing to sysfs.

Until now, to create an md device, you needed to open the relevant 
device-special file.  This created a catch-22 with udev.

This patch provides an alternate.
Options include

 echo 10 > /sys/module/md-mod/paramters/create
to create legacy device with minor 10,
 echo d10 > /sys/module/md-mod/paramters/create
to create partitionable device 10<<6
 cat /sys/module/md-mod/paramters/next_free_legacy
to return major:minor device of an unused legacy array, which will exist.
 cat /sys/module/md-mod/paramters/next_free_partitionable
to return major:minor device of an unused partitionable array which will exist.


Signed-off-by: Neil Brown <neilb@suse.de>

### Diffstat output
 ./drivers/md/md.c |   56 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 56 insertions(+)

diff ./drivers/md/md.c~current~ ./drivers/md/md.c
--- ./drivers/md/md.c~current~	2006-03-27 09:20:58.000000000 +1100
+++ ./drivers/md/md.c	2006-03-27 09:20:56.000000000 +1100
@@ -5525,6 +5525,62 @@ module_param_call(start_ro, set_ro, get_
 module_param(start_dirty_degraded, int, 0644);
 
 
+static int md_create(const char *val, struct kernel_param *kp)
+{
+	/* NN or dNN creates the numbered device */
+	int part = 0;
+	int num;
+	char *e;
+	if (*val == 'd' || *val == 'p') {
+		part = 1;
+		val++;
+	}
+	num = simple_strtoul(val, &e, 10);
+	if (*val && (*e == '\0' || *e == '\n')) {
+		/* success! */
+		dev_t dev;
+		if (part)
+			dev = MKDEV(mdp_major, num << MdpMinorShift);
+		else
+			dev = MKDEV(MD_MAJOR, num);
+		md_probe(dev, NULL, NULL);
+		return 0;
+	}
+	return -EINVAL;
+}
+static int md_next_free(char *buffer, struct kernel_param *kp)
+{
+	mddev_t *mddev;
+	int major = MD_MAJOR;
+	int inc = 1;
+	int next = MKDEV(MD_MAJOR,0);
+	if (kp->arg) {
+		next = MKDEV(mdp_major,0);
+		major = mdp_major;
+		inc = 1 << MdpMinorShift;
+	}
+	spin_lock(&all_mddevs_lock);
+	list_for_each_entry(mddev, &all_mddevs, all_mddevs)
+		if (MAJOR(mddev->unit) == major) {
+			if (atomic_read(&mddev->active)<=1 &&
+				mddev->pers == NULL &&
+				mddev->raid_disks == 0) {
+				next = mddev->unit;
+				break;
+			} else if (mddev->unit >= next)
+				next = mddev->unit + inc;
+		}
+	spin_unlock(&all_mddevs_lock);
+	md_probe(next, NULL, NULL);
+	return sprintf(buffer, "%d:%d", major, MINOR(next));
+}
+static int ignore(const char *val, struct kernel_param *kp) { return -EINVAL; }
+
+
+module_param_call(create, md_create, NULL, NULL, 0200);
+module_param_call(next_free_legacy, ignore, md_next_free, (void*)0, 0400);
+module_param_call(next_free_partitionable, ignore, md_next_free, (void*)1, 0400);
+
 EXPORT_SYMBOL(register_md_personality);
 EXPORT_SYMBOL(unregister_md_personality);
 EXPORT_SYMBOL(md_error);
-
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