Re: [PATCH] pmem: don't allocate unused major device number

From: Boaz Harrosh
Date: Sun Mar 20 2016 - 06:24:25 EST


On 03/09/2016 12:21 AM, NeilBrown wrote:
>
> When alloc_disk(0) or alloc_disk-node(0, XX) is used, the ->major
> number is completely ignored: all devices are allocated with a
> major of BLOCK_EXT_MAJOR.
>
> So there is no point allocating pmem_major.
>
> Signed-off-by: NeilBrown <neilb@xxxxxxxx>

Tested-by: Boaz Harrosh <boaz@xxxxxxxxxxxxx>

Yes I sent in the same exact patch several times. This is not the
only driver that has this "wasted code" BTW.

I hope it will be finally removed. Thanks Neil
Boaz

> ---
> drivers/nvdimm/pmem.c | 19 +------------------
> 1 file changed, 1 insertion(+), 18 deletions(-)
>
> Hi Dan et al,
> I was recently educating myself about the behavior of alloc_disk(0).
> As I understand it, the ->major is ignored and all device numbers for all
> partitions (including '0') are allocated on demand with major number of
> BLOCK_EXT_MAJOR.
>
> So I was a little surprised to find that pmem.c allocated a major
> number which is never used - historical anomaly I suspect.
> I was a bit more surprised at the comment in:
>
> Commit: 9f53f9fa4ad1 ("libnvdimm, pmem: add libnvdimm support to the pmem driver")
>
> "The minor numbers are also more predictable by passing 0 to alloc_disk()."
>
> How can they possibly be more predictable given that they are allocated
> on-demand? Maybe discovery order is very predictable???
>
> In any case, I propose this patch but cannot test it (beyond compiling)
> as I don't have relevant hardware. And maybe some user-space code greps
> /proc/devices for "pmem" to determine if "pmem" is compiled in (though
> I sincerely hope not).
> So I cannot be certain that this patch won't break anything, but am
> hoping that if you like it you might test it.
>
> If it does prove acceptable, then similar changes would be appropriate
> for btt.c and blk.c. And drivers/memstick/core/ms_block.c and
> drivers/nvme/host/core.c. (gotta stamp out this cargo cult)
>
> drivers/lightnvm/core.c is the only driver which uses alloc_disk(0) and
> doesn't provide a 'major' number. :-(
>
> Thanks,
> NeilBrown
>
>
> diff --git a/drivers/nvdimm/pmem.c b/drivers/nvdimm/pmem.c
> index 8d0b54670184..ec7e9e6a768e 100644
> --- a/drivers/nvdimm/pmem.c
> +++ b/drivers/nvdimm/pmem.c
> @@ -47,8 +47,6 @@ struct pmem_device {
> struct badblocks bb;
> };
>
> -static int pmem_major;
> -
> static bool is_bad_pmem(struct badblocks *bb, sector_t sector, unsigned int len)
> {
> if (bb->count) {
> @@ -228,8 +226,6 @@ static int pmem_attach_disk(struct device *dev,
> return -ENOMEM;
> }
>
> - disk->major = pmem_major;
> - disk->first_minor = 0;
> disk->fops = &pmem_fops;
> disk->private_data = pmem;
> disk->queue = pmem->pmem_queue;
> @@ -502,26 +498,13 @@ static struct nd_device_driver nd_pmem_driver = {
>
> static int __init pmem_init(void)
> {
> - int error;
> -
> - pmem_major = register_blkdev(0, "pmem");
> - if (pmem_major < 0)
> - return pmem_major;
> -
> - error = nd_driver_register(&nd_pmem_driver);
> - if (error) {
> - unregister_blkdev(pmem_major, "pmem");
> - return error;
> - }
> -
> - return 0;
> + return nd_driver_register(&nd_pmem_driver);
> }
> module_init(pmem_init);
>
> static void pmem_exit(void)
> {
> driver_unregister(&nd_pmem_driver.drv);
> - unregister_blkdev(pmem_major, "pmem");
> }
> module_exit(pmem_exit);
>
>