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

[Neil Brown <neilb@suse.de>]

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.

Good detective work!

NeilBrown
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