Re: [PATCH] char_dev: replace cdev_map with an xarray

[Matthew Wilcox <willy@infradead.org>]

On Mon, Jan 11, 2021 at 07:11:25PM +0100, Greg KH wrote:
> On Mon, Jan 11, 2021 at 05:35:00PM +0000, Matthew Wilcox wrote:
> > On Mon, Jan 11, 2021 at 06:05:13PM +0100, Christoph Hellwig wrote:
> > > None of the complicated overlapping regions bits of the kobj_map are
> > > required for the character device lookup, so just a trivial xarray
> > > instead.
> > 
> > Thanks for doing this.  We could make it more efficient for chardevs
> > that occupy 64 or more consecutive/aligned devices -- is it worth doing?
> 
> efficient in what way?  Space or faster lookup?

Both, but primarily space.

The radix tree underlying the xarray allows N consecutive entries with
the same value to be represented as a single entry; if there are at
least 64 entries then we get to skip an entire level of the tree (saving
1/7 of a page).  Of course, we'd need to go from the 'head' pointer to
the correct pointer, something like p += rdev - p->rdev.

> THis shouldn't be on a "fast" lookup path, so I doubt that's worth
> optimizing for.  Space, maybe, for systems with thousands of scsi
> devices, but usually they just stick to the block device, not a char
> device from what I remember.

/dev/sgX is a chardev?