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    SubjectRe: 2.6.18 ext3 panic.
    Jan Kara wrote:

    > I think it's really the 1KB block size that makes it happen.
    > I've looked at journal_dirty_data() code and I think the following can
    > happen:
    > sync() eventually ends up in journal_dirty_data(bh) as Eric writes.
    > There is finds dirty buffer attached to the comitting transaction. So it drops
    > all locks and calls sync_dirty_buffer(bh).
    > Now in other process, file is truncated so that 'bh' gets just after EOF.
    > As we have 1kb buffers, it can happen that bh is in the partially
    > truncated page. Buffer is marked unmapped and clean. But in a moment the page
    > is marked dirty and msync() is called. That eventually calls
    > set_page_dirty() and all buffers in the page are marked dirty.
    > The first process now wakes up, locks the buffer, clears the dirty bit
    > and does submit_bh() - Oops.

    Hm, just FWIW I have a couple traces* of the buffer getting unmapped
    -before- journal_submit_data_buffers ever even finds it...

    journal_submit_data_buffers():[fs/jbd/commit.c:242] needs writeout,
    adding to array pid 1836
    b_state:0x114025 b_jlist:BJ_SyncData cpu:0 b_count:2 b_blocknr:27130
    b_jbd:1 b_frozen_data:0000000000000000
    b_transaction:1 b_next_transaction:0 b_cp_transaction:0
    b_trans_is_comitting:1 b_jcount:0 pg_dirty:0

    so it's already unmapped at this point. Could
    journal_submit_data_buffers benefit from some buffer_mapped checks? Or
    is that just a bandaid too late...


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