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    SubjectRe: [3.2-rc2] loop device balance_dirty_pages_nr throttling hang
    Hi Dave,

    On Mon, Nov 21, 2011 at 10:20:56PM +0800, Dave Chinner wrote:
    > Hi Fengguang,
    >
    > I just found a way of hanging a system and taking it down. I haven't
    > tried to narrow down the test case - it's pretty simple - because it
    > time for sleep here.

    Yeah, once the global dirty limit is exceeded, the system would appear
    hang because many applications will block in balance_dirty_pages().

    I created a script for this case, however cannot reproduce it..

    The test box has 32GB memory and 110GB /dev/sda7, so I lowered
    the dirty_bytes=400MB and xfs "-b size=10g" explicitly in the script.

    During the test run on 3.2.0-rc1, I find the dirty pages rarely exceed
    the background dirty threshold (200MB).

    Would you try run this and see if this it's a problem of the test script?

    root@snb /home/wfg# cat ./test-loop-fallocate.sh
    #!/bin/sh

    # !!!change and uncomment this before run!!!
    # DEV=/dev/sda7

    echo 1 > /debug/tracing/events/writeback/balance_dirty_pages/enable
    echo 1 > /debug/tracing/events/writeback/global_dirty_state/enable

    echo $((400<<20)) > /proc/sys/vm/dirty_bytes
    mkfs.xfs -f -d size=10g $DEV
    mount $DEV /mnt/scratch
    xfs_io -f -c "truncate 20T" /mnt/scratch/scratch.img
    losetup /dev/loop0 /mnt/scratch/scratch.img
    mkfs.ext4 /dev/loop0
    mkdir /mnt/scratch/scratch
    mount /dev/loop0 /mnt/scratch/scratch
    time xfs_io -f -F -c "truncate 15T " -c "falloc 0 15T" /mnt/scratch/scratch/foo
    umount /mnt/scratch/scratch
    losetup -d /dev/loop0
    umount /mnt/scratch

    root@snb /home/wfg# ./test-loop-fallocate.sh
    meta-data=/dev/sda7 isize=256 agcount=4, agsize=655360 blks
    = sectsz=512 attr=2, projid32bit=0
    data = bsize=4096 blocks=2621440, imaxpct=25
    = sunit=0 swidth=0 blks
    naming =version 2 bsize=4096 ascii-ci=0
    log =internal log bsize=4096 blocks=2560, version=2
    = sectsz=512 sunit=0 blks, lazy-count=1
    realtime =none extsz=4096 blocks=0, rtextents=0
    mke2fs 1.42-WIP (16-Oct-2011)
    Discarding device blocks: done
    Filesystem label=
    OS type: Linux
    Block size=4096 (log=2)
    Fragment size=4096 (log=2)
    Stride=0 blocks, Stripe width=0 blocks
    335544320 inodes, 5368709120 blocks
    268435456 blocks (5.00%) reserved for the super user
    First data block=0
    163840 block groups
    32768 blocks per group, 32768 fragments per group
    2048 inodes per group
    Superblock backups stored on blocks:
    32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
    4096000, 7962624, 11239424, 20480000, 23887872, 71663616, 78675968,
    102400000, 214990848, 512000000, 550731776, 644972544, 1934917632,
    2560000000, 3855122432

    Allocating group tables: done
    Writing inode tables: done
    Creating journal (32768 blocks): done
    Writing superblocks and filesystem accounting information: done


    real 0m38.323s
    user 0m0.000s
    sys 0m25.203s

    Thanks,
    Fengguang


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