linux-kernel - Re: 2.6.18 ext3 panic.

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Date:	Tue, 10 Oct 2006 17:03:18 -0500
From:	Eric Sandeen <esandeen@...hat.com>
To:	Jan Kara <jack@...e.cz>
CC:	Dave Jones <davej@...hat.com>,
	Badari Pulavarty <pbadari@...ibm.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.

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...

-Eric

*http://people.redhat.com/esandeen/traces/eric_ext3_oops1.txt
 http://people.redhat.com/esandeen/traces/eric_ext3_oops2.txt
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