linux-kernel - Re: linux-next: build failure after merge of the ida tree

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Date:   Wed, 18 Jul 2018 15:27:46 +0200
From:   Pablo Neira Ayuso <pablo@...filter.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Stephen Rothwell <sfr@...b.auug.org.au>,
        NetFilter <netfilter-devel@...r.kernel.org>,
        Linux-Next Mailing List <linux-next@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Varsha Rao <rvarsha016@...il.com>
Subject: Re: linux-next: build failure after merge of the ida tree

On Wed, Jul 18, 2018 at 06:14:46AM -0700, Matthew Wilcox wrote:
> On Wed, Jul 18, 2018 at 11:24:26AM +0200, Pablo Neira Ayuso wrote:
> > > interacting with commit
> > > 
> > >   9679150a0bd5 ("netfilter: nf_tables: Use id allocation")
> > > 
> > > from the netfilter-next tree.
> > 
> > @Varsha, I'm very sorry, but I guess I have to toss your patch, I
> > would prefer avoid dependencies with the IDA API by now.
> 
> I've had a chance to read this patch a bit more carefully.  It transforms
> one anti-pattern into another, so I can't say I'm a fan.
> 
> The first is specific to the networking code; having a list of things
> with IDs, and constructing a bitmap when we need to allocate a new ID.
> 
> The second is having both an IDA and a list of things.
> 
> The more effective way to do all of this is to use an IDR.  You can get
> rid of the linked list *and* the IDA, and it's faster to iterate over.
> The root of the IDR is the same size as the list_head, and then you need
> only store the 4-byte ID in each element instead of the 16-byte list_head.
> 
> So Varsha, if you would like to take a look at transforming table->sets
> from a LIST_HEAD to an IDR, I think that would be a great use of your
> time.

Please, don't do so, we don't need a radix tree datastructure, it's
just more complexity.

We just wanted to have a simple way to allocate IDs using a bitmap
structure in the background without reinventing the wheel.