Re: [PATCHv4 1/1] SCSI: hosts: update to use ida_simple for host_no management

From: Lee Duncan
Date: Thu Nov 12 2015 - 11:31:59 EST

On 10/14/2015 08:53 PM, James Bottomley wrote:
> On Wed, 2015-10-14 at 11:34 -0700, Lee Duncan wrote:
>> 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.
> Um, so even if you do discovery continuously, say one every second, it
> still will take 68 years before we wrap the sign.
>> 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.
> Well, I'm a bit worried about the loss of a monotonically increasing
> host number from the debugging perspective. Right now, if you look at
> any log, hostX always refers to one and only one incarnation throughout
> the system lifetime for any given value of X. With your patch, the
> lowest host number gets continually reused ... probably for every hot
> plug event. If the USB and other hotplug system people don't mind this,
> I suppose I can live with it, but I'd like to hear their view before
> making this change.
> James


It looks like both Hannes and GregKH agreed this was the proper approach.

Lee Duncan
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