RE: [PATCH RFC 1/4] lib/radix: add universal radix_tree_fill_range

[Matthew Wilcox]

I'm not at all against the idea of having a tree which supports ranges, except that we already have one; the interval tree.  Did you investigate using the interval tree for your use case?

-----Original Message-----
From: Dan Williams [mailto:dan.j.williams@xxxxxxxxx] 
Sent: Tuesday, August 30, 2016 6:21 PM
To: Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx>
Cc: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>; Konstantin Khlebnikov <koct9i@xxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx
Subject: Re: [PATCH RFC 1/4] lib/radix: add universal radix_tree_fill_range

On Tue, Aug 30, 2016 at 3:03 PM, Ross Zwisler <ross.zwisler@xxxxxxxxxxxxxxx> 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 <mawilcox@xxxxxxxxxxxxx> 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.