On Tue, Jan 12, 2021 at 10:00:25AM +0000, David Laight wrote:
> From: Greg KH
> > Sent: 12 January 2021 09:35
> >
> > On Mon, Jan 11, 2021 at 08:58:18PM +0000, Matthew Wilcox wrote:
> > > 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 ;-(
> >
> > Looks like it will be needed here.
> >
> > Wow that's a lot of tap devices allocated all at once, odd...
>
> Aren't there two distinct use cases that probably need treating
> separately?
>
> 1) A driver that uses it's own major (private) major number.
> 2) Drivers that share a major number (eg serial ports).
>
> In the first case you just want all minors to come to your driver.
> In the second case different (ranges of) minors need to go to
> different drivers.
>
> Until the major number is actually shared only the upper limit
> is an any way relevant.
Patches gladly reviewed.
thanks,
greg k-h
