On Tue, 2006-10-10 at 17:03 -0500, Eric Sandeen wrote:
> 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_committed_data:0000000000000000
> b_transaction:1 b_next_transaction:0 b_cp_transaction:0
> b_trans_is_running: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...
Hmm..
b_state: 0x114025
^
means BH_Mapped. Isn't it ?
Thanks,
Badari
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