On Mon, Jan 11, 2021 at 06:05:13PM +0100, Christoph Hellwig wrote:
> @@ -486,14 +491,22 @@ int cdev_add(struct cdev *p, dev_t dev, unsigned count)
> if (WARN_ON(dev == WHITEOUT_DEV))
> return -EBUSY;
>
> - error = kobj_map(cdev_map, dev, count, NULL,
> - exact_match, exact_lock, p);
> - if (error)
> - return error;
> + mutex_lock(&chrdevs_lock);
> + for (i = 0; i < count; i++) {
> + error = xa_insert(&cdev_map, dev + i, p, GFP_KERNEL);
> + if (error)
> + goto out_unwind;
> + }
> + mutex_unlock(&chrdevs_lock);
Looking at some of the users ...
#define BSG_MAX_DEVS 32768
...
ret = cdev_add(&bsg_cdev, MKDEV(bsg_major, 0), BSG_MAX_DEVS);
So this is going to allocate 32768 entries; at 8 bytes each, that's 256kB.
With XArray overhead, it works out to 73 pages or 292kB. While I don't
have bsg loaded on my laptop, I imagine a lot of machines do.
drivers/net/tap.c:#define TAP_NUM_DEVS (1U << MINORBITS)
include/linux/kdev_t.h:#define MINORBITS 20
drivers/net/tap.c: err = cdev_add(tap_cdev, *tap_major, TAP_NUM_DEVS);
That's going to be even worse -- 8MB plus the overhead to be closer to 9MB.
I think we do need to implement the 'store a range' option ;-(
