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    Subjectlong sleep_on_page delays writing to slow storage
    I am running 1a67a573b (3.1.0-09125 plus a small local patch) on a Core
    i7, 8 GB RAM, writing a few GB of data to a slow SD card attached via
    usb-storage with vfat. I mounted without specifying any options,

    /dev/sdb1 /mnt/usb vfat rw,nosuid,nodev,noexec,relatime,uid=22448,gid=22448,fmask=0022,dmask=0022,codepage=cp437,iocharset=utf8,shortname=mixed,errors=remount-ro 0 0

    and I'm using rsync to write the data.

    We end up in a fairly steady state with a half GB dirty:

    Dirty: 612280 kB

    The dirty count stays high despite running sync(1) in another xterm.

    The bug is,

    Firefox (iceweasel 7.0.1-4) hangs at random intervals. One thread is
    stuck in sleep_on_page

    [<ffffffff810c50da>] sleep_on_page+0xe/0x12
    [<ffffffff810c525b>] wait_on_page_bit+0x72/0x74
    [<ffffffff811030f9>] migrate_pages+0x17c/0x36f
    [<ffffffff810fa24a>] compact_zone+0x467/0x68b
    [<ffffffff810fa6a7>] try_to_compact_pages+0x14c/0x1b3
    [<ffffffff810cbda1>] __alloc_pages_direct_compact+0xa7/0x15a
    [<ffffffff810cc4ec>] __alloc_pages_nodemask+0x698/0x71d
    [<ffffffff810f89c2>] alloc_pages_vma+0xf5/0xfa
    [<ffffffff8110683f>] do_huge_pmd_anonymous_page+0xbe/0x227
    [<ffffffff810e2bf4>] handle_mm_fault+0x113/0x1ce
    [<ffffffff8102fe3d>] do_page_fault+0x2d7/0x31e
    [<ffffffff812fe535>] page_fault+0x25/0x30
    [<ffffffffffffffff>] 0xffffffffffffffff

    And it stays stuck there for long enough for me to find the thread and
    attach strace. Apparently it was stuck in

    1320640739.201474 munmap(0x7f5c06b00000, 2097152) = 0

    for something between 20 and 60 seconds.

    There's no reason to let a 6MB/sec high latency device lock up 600 MB of
    dirty pages. I'll have to wait a hundred seconds after my app exits
    before the system will return to usability.

    And there's no way, AFAICS, for me to work around this behavior in

    And I don't understand how this compact_zone thing is intended to work
    in this situation.

    edited but nearly full dmesg at



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