On Tue, Aug 30, 2016 at 03:21:24PM -0700, Dan Williams wrote: > On Tue, Aug 30, 2016 at 3:03 PM, Ross Zwisler > <[email protected]> wrote: > > On Tue, Aug 30, 2016 at 02:56:17PM -0700, Dan Williams wrote: > >> On Mon, Aug 29, 2016 at 11:52 AM, Matthew Wilcox <[email protected]> > >> wrote: > >> > It may be protected by the mapping lock in the current code, but I would > >> > it expect it to become an RCU lookup + lock eventually. No mapping > >> > lock, just like the page cache. > >> > > >> > Even if we can work around it, why do we want to? What's the compelling > >> > reason to change from the current radix tree representation of order-N > >> > entries to an arbitrary range? There are no in-kernel users right now; > >> > is there a performance reason to change? We don't usually change an API > >> > in anticipation of future users appearing, particularly when the API > >> > makes it harder for the existing users to use it. > >> > >> I'd use a fill range api for the radix backing get_dev_pagemap() and > >> potentially another use in device-dax. It centralizes the common > >> routine of breaking down a range into its constituent power-of-2 > >> ranges. > > > > Does your usage not work with the current sibling & canonical entry model? > > It does, but I find myself writing code to walk a range and determine > the order of each entry as I insert them. I can see other users > needing the same sort of insert helper and the aspect I like of > Konstantin's proposed change is that the functionality is part of the > core implementation rather than left to be duplicated in each user.
Perhaps the answer is to have them both? Matthew's multi-order radix functionality with siblings for those of us that really *want* a single canonical entry that we can look up, use tags on, etc. And Konstantin's method where we insert a bunch of duplicate entries that don't have sibling pointers? Is there a reason why they can't coexist?
