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?
