Re: [PATCH 3/4] brd: Fix all partitions BUGs

From: Ross Zwisler
Date: Wed Aug 06 2014 - 19:07:16 EST

On Wed, 2014-08-06 at 14:33 +0300, Boaz Harrosh wrote:
> This patch fixes up brd's partitions scheme, now enjoying all worlds.
>
> The MAIN fix here is that currently if one fdisks some partitions,
> a BAD bug will make all partitions point to the same start-end sector
> ie: 0 - brd_size And an mkfs of any partition would trash the partition
> table and the other partition.
>
> Another fix is that "mount -U uuid" did not work, because of the
> GENHD_FL_SUPPRESS_PARTITION_INFO flag.
>
> So NOW the logic goes like this:
> * max_part - Just says how many minors to reserve between devices
> But in any way, there can be as many partition as requested.
> If minors between devices ends, then dynamic 259-major ids will
> be allocated on the fly.
> The default is now max_part=1, which means all partitions devt
> will be from the dynamic major-range.
> (If persistent partition minors is needed use max_part=)
>
> * Creation of new devices on the fly still/always work:
> mknod /path/devnod b 1 X
> fdisk -l /path/devnod
> Will create a new device if (X / max_part) was not already
> created before. (Just as before)
>
> partitions on the dynamically created device will work as well
> Same logic applies with minors as with the pre-created ones.
>
> TODO: dynamic grow of device size, maybe through sysfs. So each
> device can have it's own size.

With this patch we end up in what feels like a weird place where we're half
using the old scheme of major/minor allocation, and half in the world of
dynamic major/minors. Devices have a major of 1 and minors that increment by
1, but partitions have a major of 259 (BLOCK_EXT_MAJOR):

brw-rw---- 1 root disk 1, 0 Aug 6 14:10 /dev/ram0
brw-rw---- 1 root disk 1, 1 Aug 6 14:13 /dev/ram1
brw-rw---- 1 root disk 259, 0 Aug 6 14:14 /dev/ram1p1
brw-rw---- 1 root disk 259, 1 Aug 6 14:13 /dev/ram1p2
brw-rw---- 1 root disk 259, 2 Aug 6 14:14 /dev/ram1p51

For NVMe and PRD you get a major of 259 all around:

brw-rw---- 1 root disk 259, 0 Aug 6 16:55 /dev/pmem0
brw-rw---- 1 root disk 259, 2 Aug 6 16:55 /dev/pmem0p1
brw-rw---- 1 root disk 259, 3 Aug 6 16:55 /dev/pmem0p2
brw-rw---- 1 root disk 259, 1 Aug 6 16:54 /dev/pmem1

It could be that this is fine, but it just smells fishy to me I guess.

Also, it looks like you can still create a new device with this patch, but you
can't create partitions on that device. Not sure if this is just what you get
when you dynamically create a device on the fly, or if it's a symptom of
something larger.

- Ross

>
> Signed-off-by: Boaz Harrosh <boaz@xxxxxxxxxxxxx>
> ---
> drivers/block/brd.c | 93 +++++++++++++++++++++--------------------------------
> 1 file changed, 36 insertions(+), 57 deletions(-)
>
> diff --git a/drivers/block/brd.c b/drivers/block/brd.c
> index 3f07cb4..9673704 100644
> --- a/drivers/block/brd.c
> +++ b/drivers/block/brd.c
> @@ -447,16 +447,18 @@ static const struct block_device_operations brd_fops = {
> /*
> * And now the modules code and kernel interface.
> */
> -static int rd_nr;
> -int rd_size = CONFIG_BLK_DEV_RAM_SIZE;
> -static int max_part;
> -static int part_shift;
> +static int rd_nr = CONFIG_BLK_DEV_RAM_COUNT;
> module_param(rd_nr, int, S_IRUGO);
> MODULE_PARM_DESC(rd_nr, "Maximum number of brd devices");
> +
> +int rd_size = CONFIG_BLK_DEV_RAM_SIZE;
> module_param(rd_size, int, S_IRUGO);
> MODULE_PARM_DESC(rd_size, "Size of each RAM disk in kbytes.");
> +
> +static int max_part = 1;
> module_param(max_part, int, S_IRUGO);
> -MODULE_PARM_DESC(max_part, "Maximum number of partitions per RAM disk");
> +MODULE_PARM_DESC(max_part, "Num Minors to reserve between devices");
> +
> MODULE_LICENSE("GPL");
> MODULE_ALIAS_BLOCKDEV_MAJOR(RAMDISK_MAJOR);
> MODULE_ALIAS("rd");
> @@ -502,15 +504,15 @@ static struct brd_device *brd_alloc(int i)
> brd->brd_queue->limits.discard_zeroes_data = 1;
> queue_flag_set_unlocked(QUEUE_FLAG_DISCARD, brd->brd_queue);
>
> - disk = brd->brd_disk = alloc_disk(1 << part_shift);
> + disk = brd->brd_disk = alloc_disk(max_part);
> if (!disk)
> goto out_free_queue;
> disk->major = RAMDISK_MAJOR;
> - disk->first_minor = i << part_shift;
> + disk->first_minor = i * max_part;
> disk->fops = &brd_fops;
> disk->private_data = brd;
> disk->queue = brd->brd_queue;
> - disk->flags |= GENHD_FL_SUPPRESS_PARTITION_INFO;
> + disk->flags = GENHD_FL_EXT_DEVT;
> sprintf(disk->disk_name, "ram%d", i);
> set_capacity(disk, rd_size * 2);
>
> @@ -532,10 +534,11 @@ static void brd_free(struct brd_device *brd)
> kfree(brd);
> }
>
> -static struct brd_device *brd_init_one(int i)
> +static struct brd_device *brd_init_one(int i, bool *new)
> {
> struct brd_device *brd;
>
> + *new = false;
> list_for_each_entry(brd, &brd_devices, brd_list) {
> if (brd->brd_number == i)
> goto out;
> @@ -546,6 +549,7 @@ static struct brd_device *brd_init_one(int i)
> add_disk(brd->brd_disk);
> list_add_tail(&brd->brd_list, &brd_devices);
> }
> + *new = true;
> out:
> return brd;
> }
> @@ -561,70 +565,46 @@ static struct kobject *brd_probe(dev_t dev, int *part, void *data)
> {
> struct brd_device *brd;
> struct kobject *kobj;
> + bool new;
>
> mutex_lock(&brd_devices_mutex);
> - brd = brd_init_one(MINOR(dev) >> part_shift);
> + brd = brd_init_one(MINOR(dev) / max_part, &new);
> kobj = brd ? get_disk(brd->brd_disk) : NULL;
> mutex_unlock(&brd_devices_mutex);
>
> - *part = 0;
> + if (new)
> + *part = 0;
> +
> return kobj;
> }
>
> static int __init brd_init(void)
> {
> - int i, nr;
> - unsigned long range;
> struct brd_device *brd, *next;
> + int i;
>
> /*
> * brd module now has a feature to instantiate underlying device
> * structure on-demand, provided that there is an access dev node.
> - * However, this will not work well with user space tool that doesn't
> - * know about such "feature". In order to not break any existing
> - * tool, we do the following:
> *
> - * (1) if rd_nr is specified, create that many upfront, and this
> - * also becomes a hard limit.
> - * (2) if rd_nr is not specified, create CONFIG_BLK_DEV_RAM_COUNT
> - * (default 16) rd device on module load, user can further
> - * extend brd device by create dev node themselves and have
> - * kernel automatically instantiate actual device on-demand.
> + * (1) if rd_nr is specified, create that many upfront. else
> + * it defaults to CONFIG_BLK_DEV_RAM_COUNT
> + * (2) User can further extend brd devices by create dev node themselves
> + * and have kernel automatically instantiate actual device
> + * on-demand. Example:
> + * mknod /path/devnod_name b 1 X # 1 is the rd major
> + * fdisk -l /path/devnod_name
> + * If (X / max_part) was not already created it will be created
> + * dynamically.
> */
>
> - part_shift = 0;
> - if (max_part > 0) {
> - part_shift = fls(max_part);
> -
> - /*
> - * Adjust max_part according to part_shift as it is exported
> - * to user space so that user can decide correct minor number
> - * if [s]he want to create more devices.
> - *
> - * Note that -1 is required because partition 0 is reserved
> - * for the whole disk.
> - */
> - max_part = (1UL << part_shift) - 1;
> - }
> -
> - if ((1UL << part_shift) > DISK_MAX_PARTS)
> - return -EINVAL;
> -
> - if (rd_nr > 1UL << (MINORBITS - part_shift))
> - return -EINVAL;
> -
> - if (rd_nr) {
> - nr = rd_nr;
> - range = rd_nr << part_shift;
> - } else {
> - nr = CONFIG_BLK_DEV_RAM_COUNT;
> - range = 1UL << MINORBITS;
> - }
> -
> if (register_blkdev(RAMDISK_MAJOR, "ramdisk"))
> return -EIO;
>
> - for (i = 0; i < nr; i++) {
> + if (unlikely(!max_part))
> + max_part = 1;
> +
> + for (i = 0; i < rd_nr; i++) {
> brd = brd_alloc(i);
> if (!brd)
> goto out_free;
> @@ -636,7 +616,7 @@ static int __init brd_init(void)
> list_for_each_entry(brd, &brd_devices, brd_list)
> add_disk(brd->brd_disk);
>
> - blk_register_region(MKDEV(RAMDISK_MAJOR, 0), range,
> + blk_register_region(MKDEV(RAMDISK_MAJOR, 0), 1UL << MINORBITS,
> THIS_MODULE, brd_probe, NULL, NULL);
>
> printk(KERN_INFO "brd: module loaded\n");
> @@ -649,21 +629,20 @@ out_free:
> }
> unregister_blkdev(RAMDISK_MAJOR, "ramdisk");
>
> + printk(KERN_INFO "brd: module NOT loaded !!!\n");
> return -ENOMEM;
> }
>
> static void __exit brd_exit(void)
> {
> - unsigned long range;
> struct brd_device *brd, *next;
>
> - range = rd_nr ? rd_nr << part_shift : 1UL << MINORBITS;
> -
> list_for_each_entry_safe(brd, next, &brd_devices, brd_list)
> brd_del_one(brd);
>
> - blk_unregister_region(MKDEV(RAMDISK_MAJOR, 0), range);
> + blk_unregister_region(MKDEV(RAMDISK_MAJOR, 0), 1UL << MINORBITS);
> unregister_blkdev(RAMDISK_MAJOR, "ramdisk");
> + printk(KERN_INFO "brd: module unloaded\n");
> }
>
> module_init(brd_init);




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