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

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Message-ID: <20160830220030.GA20121@linux.intel.com>
Date:   Tue, 30 Aug 2016 16:00:30 -0600
From:   Ross Zwisler <ross.zwisler@...ux.intel.com>
To:     Matthew Wilcox <mawilcox@...rosoft.com>
Cc:     Konstantin Khlebnikov <koct9i@...il.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.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 Mon, Aug 29, 2016 at 06:52:54PM +0000, Matthew Wilcox 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 do have a patch set out for review which uses the multi-order nature of the
radix tree:

https://lkml.org/lkml/2016/8/23/725

This code takes advantage of the fact that using the radix tree for an order-0
entry is the same as for a multi-order entry.  Both have a single lock bit,
and a single entry that i need to use for lookups, sets, locking and
unlocking.

My usage fits well with the current implementation of the radix tree, and I'd
like to keep it simple if I can.