From mboxrd@z Thu Jan 1 00:00:00 1970 From: Lee Duncan Subject: Re: [PATCHv4 1/1] SCSI: hosts: update to use ida_simple for host_no management Date: Wed, 14 Oct 2015 11:34:00 -0700 Message-ID: <561EA018.7020700@suse.com> References: <1444830904.2220.28.camel@HansenPartnership.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Return-path: In-Reply-To: <1444830904.2220.28.camel@HansenPartnership.com> Sender: linux-kernel-owner@vger.kernel.org To: James Bottomley Cc: linux-scsi@vger.kernel.org, linux-kernel@vger.kernel.org, Tejun Heo , Hannes Reinecke , Johannes Thumshirn , Christoph Hellwig List-Id: linux-scsi@vger.kernel.org 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