Re: dm-io async WRITE_SAME results in iSCSI NULL pointer [was: Re: Write same support]

[Mike Christie <michaelc@cs.wisc.edu>]

On 02/16/2012 03:03 PM, Mike Snitzer wrote:
> On Thu, Feb 16 2012 at  3:02pm -0500,
> Mike Snitzer <snitzer@redhat.com> wrote:
> 
>> FYI, I'll bounce a message detailing the iSCSI scatter-gather NULL
>> pointer I _always_ hit with dm-io issuing async WRITE_SAME.
> 
> I developed a patch for dm-io so that the new dm-thinp target can
> leverage your new WRITE SAME functionality for, hopefully, more
> efficient zeroing of the disk (see: dm-io-WRITE_SAME.patch at the end of
> the following patchset).
> 
> Here is the patchset I'm using ontop of Linux 3.2:
> http://people.redhat.com/msnitzer/patches/upstream/dm-io-WRITE_SAME/series.html
> 
> All works great on FC (tested against NetApp 3040 LUN)... I'm using the
> thinp-test-suite to test dm-thinp's use of dm_kcopyd_zero().
> 
> But testing with iSCSI, I get a NULL pointer _every_ time in the iSCSI
> scatter-gather code, see:
> http://people.redhat.com/msnitzer/patches/upstream/dm-io-WRITE_SAME/async-WRITE_SAME-makes-iscsi-sg-die.txt
> -- in the middle of that file you'll see my 'crash' analysis of the
> issue -- but that is just the NULL pointer.. no idea what the smoking
> gun is that caused the iscsi_segment to become NULL.
> 
> Anyway, taking a step back... WRITE SAME is all about transfering a
> single logical block, backed by a single empty_zero_page in this test
> case, so I'm wondering if for some reason iSCSI's sg code is getting
> confused and thinking that more pages need to be transferred than were
> in the original bio's payload (but iSCSI is way beneath the bio -> SCSI
> command translation... grr)

Yeah, probably a request/scsi_cmnd/sg sector/length/offset value is off
or iscsi is making a bad assumption.

Do:

echo 1 > /sys/module/libiscsi/parameters/debug_libiscsi_session
echo 1 > /sys/module/libiscsi/parameters/debug_libiscsi_session
echo 1 > /sys/module/libiscsi_tcp/parameters/debug_libiscsi_tcp
echo 1 > /sys/module/libiscsi_tcp/parameters/iscsi_tcp

then rerun your test.