15 partitions (at least for sd_mod devices) are too few.

So I tried the following: after scanning the disk (sda), when we know
the number of partitions P on a disk, create a new block device
/dev/gd0 that is a copy of sda (in terms of disk->queue, etc.). This
is done using alloc_disk(P).

However, read() on gd0 will just return 0. It takes a `blockdev --rereadpt
/dev/gd0` before the disk is accessible. And if I add a call to
rescan inside gpdisk_new() it oopses (probably rightfully so). I do not 
know all the block layer magic, so expect some horrible code.

Ideas, hints, anything is welcome.

---
 block/Makefile        |    1 
 block/genhd.c         |    5 +
 block/gpdisk.c        |  139 ++++++++++++++++++++++++++++++++++++++++++++++++++
 drivers/scsi/sd.c     |    1 
 fs/partitions/check.c |   19 ++++++
 include/linux/genhd.h |   16 +++++
 include/scsi/sd.h     |    3 +
 7 files changed, 182 insertions(+), 2 deletions(-)

Index: linux-2.6.23/block/Makefile
===================================================================
--- linux-2.6.23.orig/block/Makefile
+++ linux-2.6.23/block/Makefile
@@ -3,6 +3,7 @@
 #
 
 obj-$(CONFIG_BLOCK) := elevator.o ll_rw_blk.o ioctl.o genhd.o scsi_ioctl.o
+obj-${CONFIG_BLOCK} += gpdisk.o
 
 obj-$(CONFIG_BLK_DEV_BSG)	+= bsg.o
 obj-$(CONFIG_IOSCHED_NOOP)	+= noop-iosched.o
Index: linux-2.6.23/block/genhd.c
===================================================================
--- linux-2.6.23.orig/block/genhd.c
+++ linux-2.6.23/block/genhd.c
@@ -744,6 +744,9 @@ struct gendisk *alloc_disk_node(int mino
 		rand_initialize_disk(disk);
 		INIT_WORK(&disk->async_notify,
 			media_change_notify_thread);
+		atomic_set(&disk->gpdisk_enabled, false);
+		disk->gpdisk         = NULL;
+		disk->gpdisk_parent  = NULL;
 	}
 	return disk;
 }
@@ -793,6 +796,8 @@ EXPORT_SYMBOL(set_device_ro);
 void set_disk_ro(struct gendisk *disk, int flag)
 {
 	int i;
+	if (gpdisk_online(disk))
+		set_disk_ro(disk->gpdisk, flag);
 	disk->policy = flag;
 ...
[ message continues ]

Now when we have 20-bit minors, can't we simply recycle some of the 
higher bits for additional partitions, across the board?  63 partitions 
seem to have been sufficient; at least I haven't heard anyone complain 
about that for 15 years.

	-hpa
-

[ message continues ]

GPT allows up to 128 partitions, and the linux partition code currently 
allows for up to MAX_PART (256). Assuming 1048576/128, that would give 
8192 disks. With dynamic minor allocation and reuse, all that goes away 
and the limit becomes a bit less than 1048576 _partitions_.
-

[ message continues ]

Yes, but you're proposing something with substantially higher switch 
threshold...

	-hpa
-

[ message continues ]

On Fri, 05 Oct 2007 15:11:52 -0700

This was proposed ages ago. Al Viro vetoed sparse minors and it has been
stuck this way ever since. If you have > 15 partitions use device mapper
for it. I'd prefer it fixed but its arguable that device mapper is the
right way to punt all our partitioning to userspace


Alan
-

[ message continues ]

Sure.  However, that takes having that bit of userspace in even the most 
trivial configurations, and not just on bootup, but continuously.

	-hpa
-

[ message continues ]

I'm not sure that configurations requiring more than 15 partitions are 
properly described as "trivial." Which is not to disagree with your 
point about required user tools, but most systems needing such tools 
will be large and complex enough that a userspace solution will be 
acceptable.


-- 
Bill Davidsen <davidsen@tmr.com>
   "We have more to fear from the bungling of the incompetent than from
the machinations of the wicked."  - from Slashdot
-

[ message continues ]

And there is LVM too, which seems better than partitions in many cases.

--
Len Sorensen
-

[ message continues ]

Then please fix support for extended partitions in kpartx (part of 
multipath-tools). Debian has an incomplete patch that does the right 
thing on activation, but not on deactivation of partitions, and has an 
obvious off-by-one in the "kpartx -l /dev/sda" output.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Edited by Alexander E. Patrakov to fix incorrect output of "kpartx -l"
Signed-off-by: Alexander E. Patrakov <patrakov@ums.usu.ru>

--- a/kpartx/kpartx.c
+++ b/kpartx/kpartx.c
@@ -387,10 +387,10 @@ main(int argc, char **argv){
 					slices[j].minor = m++;
 
 					start = slices[j].start - slices[k].start;
-					printf("%s%s%d : 0 %lu /dev/dm-%d %lu\n",
+					printf("%s%s%d : 0 %lu %s%s%d %lu\n",
 					       mapname, delim, j+1,
 					       (unsigned long) slices[j].size,
-					       slices[k].minor, start);
+					       mapname, delim, k+1, start);
 					c--;
 				}
 				/* Terminate loop if nothing more to resolve */
@@ -431,7 +431,7 @@ main(int argc, char **argv){
 			break;
 
 		case ADD:
-			for (j=0, c = 0; j<n; j++) {
+			for (j = 0, c = 0; j < n; j++) {
 				if (slices[j].size == 0)
 					continue;
 
@@ -477,6 +477,7 @@ main(int argc, char **argv){
 			d = c;
 			while (c) {
 				for (j = 0; j < n; j++) {
+					unsigned long start;
 					int k = slices[j].container - 1;
 
 					if (slices[j].size == 0)
@@ -487,7 +488,7 @@ main(int argc, char **argv){
 						continue;
 
 					/* Skip all simple slices */
-					if (k < 0)
+					if (slices[j].container == 0)
 						continue;
 
 					/* Check container slice */
@@ -502,10 +503,11 @@ main(int argc, char **argv){
 					}
 					strip_slash(partname);
 
+					start = slices[j].start - slices[k].start;
 					if (safe_sprintf(params, "%d:%d %lu",
 							 slices[k].major,
 							 slices[k].minor,
-							 (unsigned long)slices[j].start)) {
+							 start)) {
 						fprintf(stderr, "params too small\n");
 						exit(1);
 					}
@@ -524,9 +526,12 @@ main(int argc, char **argv){
 ...
[ message continues ]