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    SubjectRe: fs: GPF in locked_inode_to_wb_and_lock_list
    Hello, Dmitry.

    On Mon, Apr 18, 2016 at 11:44:11AM +0200, Dmitry Vyukov wrote:

    > general protection fault: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN
    ...
    > RIP: 0010:[<ffffffff818884d2>] [<ffffffff818884d2>]
    > locked_inode_to_wb_and_lock_list+0xa2/0x750
    > RSP: 0018:ffff88006cdaf7d0 EFLAGS: 00010246
    > RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffff88006ccf2050
    > RDX: 0000000000000000 RSI: 000000114c8a8484 RDI: 0000000000000286
    > RBP: ffff88006cdaf820 R08: ffff88006ccf1840 R09: 0000000000000000
    > R10: 000229915090805f R11: 0000000000000001 R12: ffff88006a72f5e0
    > R13: dffffc0000000000 R14: ffffed000d4e5eed R15: ffffffff8830cf40
    > FS: 0000000000000000(0000) GS:ffff88006d500000(0000) knlGS:0000000000000000
    > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
    > CR2: 0000000003301bf8 CR3: 000000006368f000 CR4: 00000000000006e0
    > DR0: 0000000000001ec9 DR1: 0000000000000000 DR2: 0000000000000000
    > DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000600
    > Stack:
    > ffff88006a72f680 ffff88006a72f768 ffff8800671230d8 03ff88006cdaf948
    > ffff88006a72f668 ffff88006a72f5e0 ffff8800671230d8 ffff88006cdaf948
    > ffff880065b90cc8 ffff880067123100 ffff88006cdaf970 ffffffff8188e12e
    > Call Trace:
    > [< inline >] inode_to_wb_and_lock_list fs/fs-writeback.c:309
    > [<ffffffff8188e12e>] writeback_sb_inodes+0x4de/0x1250 fs/fs-writeback.c:1554
    > [<ffffffff8188efa4>] __writeback_inodes_wb+0x104/0x1e0 fs/fs-writeback.c:1600
    > [<ffffffff8188f9ae>] wb_writeback+0x7ce/0xc90 fs/fs-writeback.c:1709
    > [< inline >] wb_do_writeback fs/fs-writeback.c:1844
    > [<ffffffff81891079>] wb_workfn+0x2f9/0x1000 fs/fs-writeback.c:1884
    > [<ffffffff813bcd1e>] process_one_work+0x78e/0x15c0 kernel/workqueue.c:2094
    > [<ffffffff813bdc2b>] worker_thread+0xdb/0xfc0 kernel/workqueue.c:2228
    > [<ffffffff813cdeef>] kthread+0x23f/0x2d0 drivers/block/aoe/aoecmd.c:1303
    > [<ffffffff867bc5d2>] ret_from_fork+0x22/0x50 arch/x86/entry/entry_64.S:392
    > Code: 05 94 4a a8 06 85 c0 0f 85 03 03 00 00 e8 07 15 d0 ff 41 80 3e
    > 00 0f 85 64 06 00 00 49 8b 9c 24 88 01 00 00 48 89 d8 48 c1 e8 03 <42>
    > 80 3c 28 00 0f 85 17 06 00 00 48 8b 03 48 83 c0 50 48 39 c3
    > RIP [< inline >] wb_get include/linux/backing-dev-defs.h:212
    > RIP [<ffffffff818884d2>] locked_inode_to_wb_and_lock_list+0xa2/0x750
    > fs/fs-writeback.c:281
    > RSP <ffff88006cdaf7d0>

    Man, that's a beautiful trace w/ decoding of inline functions. When
    did we start doing that? Is there a specific config option for this?

    > ---[ end trace 986a4d314dcb2694 ]---
    > The crash happened here:
    >
    > if (wb != &wb->bdi->wb)
    > ffffffff818884cb: 48 89 d8 mov %rbx,%rax
    > ffffffff818884ce: 48 c1 e8 03 shr $0x3,%rax
    > ffffffff818884d2: 42 80 3c 28 00 cmpb $0x0,(%rax,%r13,1)

    So, it's the above instruction.

    > ffffffff818884d7: 0f 85 17 06 00 00 jne
    > ffffffff81888af4 <locked_inode_to_wb_and_lock_list+0x6c4>
    > ffffffff818884dd: 48 8b 03 mov (%rbx),%rax
    > ffffffff818884e0: 48 83 c0 50 add $0x50,%rax
    > ffffffff818884e4: 48 39 c3 cmp %rax,%rbx
    > ffffffff818884e7: 0f 84 c3 00 00 00 je
    > ffffffff818885b0 <locked_inode_to_wb_and_lock_list+0x180>
    >
    > Which means that bdi is NULL (if I get indirections right).

    So, the wb != &wb->bdi->wb comparison would be the cmp at
    0xffffffff818884e4 and given that it just compares the address of
    &bdi->wb, bdi being NULL wouldn't trigger the fault.

    cmpb $0x0,(%rax,%r13,1)
    -> *(u8 *)(%rax + %r13) == 0
    -> *(u8 *)((%rbx >> 3) + %r13) == 0

    Where can that be from? I can't find anything matching even in the
    surrounding functions.

    Hmmm... The base address %r13 is 0xdffffc0000000000 which isn't a
    proper canonical address and in general suspcious. Ooh, it's
    KASAN_SHADOW_OFFSET. It looks like something is making KASAN trigger
    a fault. Can we please bring in someone who's more familiar with
    KASAN?

    Thanks.

    --
    tejun

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