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