Message-ID: <1411082258.28382.17.camel@haakon3.risingtidesystems.com>
Date:	Thu, 18 Sep 2014 16:17:38 -0700
From:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:	Andy Grover <agrover@...hat.com>
Cc:	Christoph Hellwig <hch@....de>, target-devel@...r.kernel.org,
	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCHv3 0/8] target: Save memory on unused se_dev_entrys and
 se_luns

On Thu, 2014-09-18 at 15:54 -0700, Andy Grover wrote:
> On 09/18/2014 12:38 AM, Nicholas A. Bellinger wrote:
> > On Sat, 2014-09-13 at 21:55 +0200, Christoph Hellwig wrote:
> >> ping again.  We're getting closer to the end of the 3.18 merge window
> >> and there still hasn't been a response.  Should Andy just send the patches
> >> directly to Linus once 3.18 opens given that they have been out on the list
> >> since Jun 23? (with a positive review from me and no negative one)
> 
> > Removing unused per WWPN endpoint LUN + per NodeACL MappedLUN memory is
> > a nice optimization to have, but I'm not yet convinced that extending
> > existing control path spinlocks to support an array of pointers is
> > ultimately worth the complexity it adds here.
> 
> 9 files changed, 250 insertions(+), 367 deletions(-).
> 
> This patchset removes 100+ lines of code. Furthermore, I wouldn't 
> characterize it as extending locks, so much as putting locks where they 
> should've always been. The fact that device_list[foo] is never null 
> means we've avoided crashes but not potentially incorrect accesses.
> 

It's the extending of locks over other locks that make aspects of this
series problematic to existing code.  Please run them with lock
debugging on and perform a few active I/O shutdowns and you'll see what
I mean.

Not to mention the patches with the locking changes mix cleanups with
bug-fixes, which I really dislike when reviewing patches of this nature.

> > Another concern is how these changes effect active session + device I/O
> > shutdown, which is an area of regressions I'd rather avoid
> 
> I assume this set would spend time in your tree, followed by Linus' tree 
> before making it into a release. Also, any logic errors are likely to 
> result in a fault, so they should not remain hidden for long.
> 
> > if the
> > primary benefit of this series is only reducing memory footprint for
> > unused LUNs + MappedLUNs.
> 
> Yes it does reduce wasted memory, that should be reason enough I'd say. 
> But this patchset is also a building block for further improvements that 
> are more significant. This set transitions all lun and mappedlun checks 
> from checking a flag to checking for NULL. This is necessary before we 
> can improve from a fixed-size array to more size-scalable data 
> structures like a radix tree, or lockless, with RCU.

Like I said, I don't agree this is the best way to get to something
smarter.

> 
> >  Lowering the TRANSPORT_MAX_LUNS_PER_TPG value
> > at compile time today is the simple way for reducing overall memory
> > footprint for folks who need to scale up the number of targets using
> > smaller individual LUN mappings.
> 
> This is only an option for embedded. We should scale the amount of 
> memory we use with the number of allocated LUNs and mapped LUNs.
> 
> > As for something smarter, given the mostly read-only nature of LUN +
> > MappedLUN pointers to individual TPGT + NodeACLs context, I'd rather see
> > something based on RCU arrays + percpu_ref counting to avoid this type
> > of complexity to existing code, and move in the direction of dropping
> > fast-path I_T ->device_list_lock access all together.
> 
> See above about pointers vs flags, this is a first step toward more 
> performant *and* space-efficient data structures.
> 
> > Beyond these objections, there are some useful fixes + cleanups from
> > this series that I'm OK with merging soon..
> 
> I've pushed this patchset to
> 
> git://git.kernel.org/pub/scm/linux/kernel/git/grover/linux.git
> 
> on two branches against your and Linus' repos:
> against-linus
> against-target-pending-for-next
> 
> (looked-over and compile-tested)
> 

I'm currently reviewing #2, #4, #5 and #7 and will consider merging
these.  These cleanups account for most of the LOC reduction, and avoid
most of the larger concerns.

Also for patches like this, they really need testing on your end before
I'll consider them merging.  Compile testing alone for these types of
locking changes is not enough, given the history of regressions with
these types of cleanups.

Thanks,

--nab

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