linux-kernel - Re: [PATCH RFC 1/4] lib/radix: add universal radix_tree_fill

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Date:   Tue, 30 Aug 2016 16:53:16 -0600
From:   Ross Zwisler <ross.zwisler@...ux.intel.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Ross Zwisler <ross.zwisler@...ux.intel.com>,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Konstantin Khlebnikov <koct9i@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC 1/4] lib/radix: add universal radix_tree_fill_range

On Tue, Aug 30, 2016 at 03:21:24PM -0700, Dan Williams wrote:
> On Tue, Aug 30, 2016 at 3:03 PM, Ross Zwisler
> <ross.zwisler@...ux.intel.com> 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@...rosoft.com> 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?