Re: More information on scsi_cmd_cache leak... (bisect)

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

Jens Axboe wrote:
> On Fri, Jan 27 2006, Jens Axboe wrote:
> 
>>On Fri, Jan 27 2006, Neil Brown wrote:
>>
>>>On Friday January 27, chase.venters@clientec.com wrote:
>>>
>>>>Greetings,
>>>>	Just a quick recap - there are at least 4 reports of 2.6.15 users 
>>>>experiencing severe slab leaks with scsi_cmd_cache. It seems that a few of us 
>>>>have a board (Asus P5GDC-V Deluxe) in common. We seem to have raid in common. 
>>>>	After dealing with this leak for a while, I decided to do some dancing around 
>>>>with git bisect. I've landed on a possible point of regression:
>>>>
>>>>commit: a9701a30470856408d08657eb1bd7ae29a146190
>>>>[PATCH] md: support BIO_RW_BARRIER for md/raid1
>>>>
>>>>	I spent about an hour and a half reading through the patch, trying to see if 
>>>>I could make sense of what might be wrong. The result (after I dug into the 
>>>>code to make a change I foolishly thought made sense) was a hung kernel.
>>>>	This is important because when I rebooted into the kernel that had been 
>>>>giving me trouble, it started an md resync and I'm now watching (at least 
>>>>during this resync) the slab usage for scsi_cmd_cache stay sane:
>>>>
>>>>turbotaz ~ # cat /proc/slabinfo | grep scsi_cmd_cache
>>>>scsi_cmd_cache        30     30    384   10    1 : tunables   54   27    8 : 
>>>>slabdata      3      3      0
>>>>
>>>
>>>This suggests that the problem happens when a BIO_RW_BARRIER write is
>>>sent to the device.  With this patch, md flags all superblock writes
>>>as BIO_RW_BARRIER However md is not so likely to update the superblock often
>>>during a resync.
>>>
>>>There is a (rough) count of the number of superblock writes in the
>>>"Events" counter which "mdadm -D" will display.
>>>You could try collecting 'Events' counter together with the
>>>'active_objs' count from /proc/slabinfo and graph the pairs - see if
>>>they are linear.
>>>
>>>I believe a BIO_RW_BARRIER is likely to send some sort of 'flush'
>>>command to the device, and the driver for your particular device may
>>>well be losing scsi_cmd_cache allocation when doing that, but I leave
>>>that to someone how knows more about that code.
>>
>>I already checked up on that since I suspected barriers initially. The
>>path there for scsi is sd.c:sd_issue_flush() which looks pretty straight
>>forward. In the end it goes through the block layer and gets back to the
>>SCSI layer as a regular REQ_BLOCK_PC request.
> 
> 
> Sorry, that was for the ->issue_flush() that md also does but did before
> the barrier addition as well. Most of the barrier handling is done in
> the block layer, but it could show leaks in SCSI of course. FWIW, I
> tested barriers with and without md on SCSI here a few days ago and
> didn't see any leaks at all.
> 

It does not have anything to do with this in scsi_io_completion does it?

         if (blk_complete_barrier_rq(q, req, good_bytes >> 9))
                 return;

For that case the scsi_cmnd does not get freed. Does it come back around 
again and get released from a different path?