linux-kernel - Re: 2.6.18 ext3 panic.

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Date:	Tue, 10 Oct 2006 15:25:06 -0700
From:	Badari Pulavarty <pbadari@...ibm.com>
To:	Eric Sandeen <esandeen@...hat.com>
Cc:	Jan Kara <jack@...e.cz>, Dave Jones <davej@...hat.com>,
	Eric Sandeen <sandeen@...deen.net>,
	Andrew Morton <akpm@...l.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: 2.6.18 ext3 panic.

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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