Re: 2.6.18 ext3 panic.

[Eric Sandeen <sandeen@sandeen.net>]

Badari Pulavarty wrote:
> 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 ?

Whoops, I pasted in the wrong one, I guess, from earlier in the trace.  Here are 
the ones I was looking at:

 journal_submit_data_buffers():[fs/jbd/commit.c:242] needs writeout, adding to 
array pid 1690
     b_state:0x104005 b_jlist:BJ_SyncData cpu:0 b_count:2 b_blocknr:30045
     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:1

and

 journal_submit_data_buffers():[fs/jbd/commit.c:242] needs writeout, adding to 
array pid 1836
     b_state:0x114005 b_jlist:BJ_SyncData cpu:1 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:1

-Eric
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