linux-kernel - Re: [dm-devel] The DM_BUFIO_HASH_BITS is defined as 20, any plan to make it flexible?

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Date:	Wed, 3 Dec 2014 14:19:22 +0000
From:	Joe Thornber <thornber@...hat.com>
To:	device-mapper development <dm-devel@...hat.com>
Cc:	zhangxiao <xiao.zhang@...driver.com>, linux-raid@...r.kernel.org,
	linux-kernel@...r.kernel.org, agk@...hat.com
Subject: Re: [dm-devel] The DM_BUFIO_HASH_BITS is defined as 20, any plan to
 make it flexible?

On Tue, Dec 02, 2014 at 09:00:17AM -0500, Mike Snitzer wrote:
> On Tue, Dec 02 2014 at  3:43am -0500,
> zhangxiao <xiao.zhang@...driver.com> wrote:
> 
> > Hi Experts,
> > 
> > See drivers/md/dm-bufio.c as below:
> >   50 /*
> >   51  * Buffer hash
> >   52  */
> >   53 #define DM_BUFIO_HASH_BITS      20
> >   54 #define DM_BUFIO_HASH(block) \
> >   55         ((((block) >> DM_BUFIO_HASH_BITS) ^ (block)) & \
> >   56          ((1 << DM_BUFIO_HASH_BITS) - 1))
> > "drivers/md/dm-bufio.c" 1854L, 45375C
> > 
> > DM_BUFIO_HASH_BITS was defined as 20 and it maybe not fit for some
> > use case with limited resources. Do we have any plan to make it more
> > flexible? Like module parameter or something else?
> 
> No immediate plans.

That hash table has been replaced with a red/black tree.

https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-for-3.19&id=4e420c452b11edf9d510c8180ac66f529e5b6206

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