linux-kernel - Re: [PATCHv4 1/1] SCSI: hosts: update to use ida_simple for host

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Date:	Wed, 14 Oct 2015 11:34:00 -0700
From:	Lee Duncan <lduncan@...e.com>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
	Tejun Heo <tj@...nel.org>, Hannes Reinecke <hare@...e.com>,
	Johannes Thumshirn <jthumshirn@...e.de>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCHv4 1/1] SCSI: hosts: update to use ida_simple for host_no
 management

On 10/14/2015 06:55 AM, James Bottomley wrote:
> On Wed, 2015-10-07 at 16:51 -0700, Lee Duncan wrote:
>> Update the SCSI hosts module to use the ida_simple*() routines
>> to manage its host_no index instead of an ATOMIC integer. This
>> means that the SCSI host number will now be reclaimable.
> 
> OK, but why would we want to do this?  We do it for sd because our minor
> space for the device nodes is very constrained, so packing is essential.
> For HBAs, there's no device space density to worry about, they're
> largely statically allocated at boot time and not reusing the numbers
> allows easy extraction of hotplug items for the logs (quite useful for
> USB) because each separate hotplug has a separate and monotonically
> increasing host number.
> 
> James
> 

Good question, James. Apologies for not making the need clear.

The iSCSI subsystem uses a host structure for discovery, then throws it
away. So each time it does discovery it gets a new host structure. With
the current approach, that number is ever increasing. It's only a matter
of time until some user with a hundreds of disks and perhaps thousands
of LUNs, that likes to do periodic discovery (think super-computers)
will run out of host numbers. Or, worse yet, get a negative number
number (because the value is signed right now).

And this use case is a real one right now, by the way.

As you can see from the patch, it's a small amount of code to ensure
that the host number management is handled more cleanly.

-- 
Lee Duncan

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