linux-kernel - Re: [PATCH] char_dev: replace cdev

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Date:   Tue, 12 Jan 2021 11:25:35 +0100
From:   'Greg KH' <gregkh@...uxfoundation.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Christoph Hellwig <hch@....de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] char_dev: replace cdev_map with an xarray

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