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    SubjectRe: More information on scsi_cmd_cache leak... (bisect)
    On Friday January 27, 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!

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      Last update: 2006-01-27 12:14    [W:0.026 / U:6.688 seconds]
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