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    Subjectpagefault in generic_file_buffered_write() causing deadlock
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    Hi Andrew & MM experts,

    We are looking at a customer situation (on 2.6.16-based distro) - where
    system becomes almost useless while running some java & stress tests.

    Root cause seems to be taking a pagefault in generic_file_buffered_write
    () after calling prepare_write. I am wondering

    1) Why & How this can happen - since we made sure to fault the user
    buffer before prepare write.

    2) If this is already fixed in current mainline (I can't see how).

    Ideas on what I can do to fix it ?

    Thanks,
    Badari

    Here is the analysis & stacks:
    ===============================

    Java thread doing mmap() holding for mmap_sem and waiting for
    transaction to be unlocked:

    java D 000000000fed3ff4 7104 2447 2391 2448 2446
    (NOTLB)
    Call Trace:
    [C00000002AC8F410] [C000000001315AC0] 0xc000000001315ac0 (unreliable)
    [C00000002AC8F5E0] [C00000000000F0B4] .__switch_to+0x12c/0x150
    [C00000002AC8F670] [C00000000039980C] .schedule+0xcec/0xe4c
    [C00000002AC8F780] [C00000000017BC24] .start_this_handle+0x3b4/0x4ac
    [C00000002AC8F8A0] [C00000000017BE08] .journal_start+0xec/0x140
    [C00000002AC8F940] [C000000000171374] .ext3_journal_start_sb+0x58/0x78
    [C00000002AC8F9C0] [C00000000016AB90] .ext3_dirty_inode+0x38/0xb0
    [C00000002AC8FA50] [C0000000000F6820] .__mark_inode_dirty+0x60/0x1d4
    [C00000002AC8FAF0] [C0000000000E9F60] .touch_atime+0xc8/0xe0
    [C00000002AC8FB80] [C000000000093834] .generic_file_mmap+0x54/0x80
    [C00000002AC8FC00] [C0000000000AC450] .do_mmap_pgoff+0x558/0x870
    [C00000002AC8FD10] [C00000000000A9C0] .sys_mmap+0xdc/0x160
    [C00000002AC8FDC0] [C000000000014258] .compat_sys_mmap2+0x14/0x28
    [C00000002AC8FE30] [C00000000000871C] syscall_exit+0x0/0x40


    kjournald locked the transaction and waiting for journal stop
    (t_updates to go to zero):

    kjournald D 0000000000000000 8704 2167 1 2203 2028
    (L-TLB)
    Call Trace:
    [C00000003514F980] [C0000000005257D8] amd74xx_pci_tbl+0x8/0x200 (unreliable)
    [C00000003514FB50] [C00000000000F0B4] .__switch_to+0x12c/0x150
    [C00000003514FBE0] [C00000000039980C] .schedule+0xcec/0xe4c
    [C00000003514FCF0] [C00000000017DA58] .journal_commit_transaction+0x190/0x1448
    [C00000003514FE50] [C000000000182F44] .kjournald+0xf0/0x27c
    [C00000003514FF90] [C000000000025630] .kernel_thread+0x4c/0x68

    Another java thread, did journal_start() in prepare_write() and
    took a pagefault while copying. Now this is waiting for mmap_sem
    to finish the fault :(

    java D 000000000ffd76f0 6384 2452 2391 2453 2451
    (NOTLB)
    Call Trace:
    [C00000002ABBEE50] [C00000002ABBEEE0] 0xc00000002abbeee0 (unreliable)
    [C00000002ABBF020] [C00000000000F0B4] .__switch_to+0x12c/0x150
    [C00000002ABBF0B0] [C00000000039980C] .schedule+0xcec/0xe4c
    [C00000002ABBF1C0] [C00000000039B688] .rwsem_down_read_failed
    +0x284/0x2d0
    [C00000002ABBF290] [C00000000039D58C] .do_page_fault+0x2e4/0x75c
    [C00000002ABBF460] [C000000000004860] .handle_page_fault+0x20/0x54
    --- Exception: 301 at .__copy_tofrom_user+0x11c/0x580
    LR = .generic_file_buffered_write+0x39c/0x7c8
    [C00000002ABBF750] [C000000000095A94]
    .generic_file_buffered_write+0x2c0/0x7c8 (
    unreliable)
    [C00000002ABBF8F0] [C0000000000962EC]
    .__generic_file_aio_write_nolock+0x350/0x3
    e0
    [C00000002ABBFA20] [C000000000096908] .generic_file_aio_write+0x78/0x104
    [C00000002ABBFAE0] [C0000000001649F0] .ext3_file_write+0x2c/0xd4
    [C00000002ABBFB70] [C0000000000C5168] .do_sync_write+0xd4/0x130
    [C00000002ABBFCF0] [C0000000000C5ED4] .vfs_write+0x128/0x20c
    [C00000002ABBFD90] [C0000000000C664C] .sys_write+0x4c/0x8c
    [C00000002ABBFE30] [C00000000000871C] syscall_exit+0x0/0x40


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