From: Alberto Bertogli <albertito@blitiri.com.ar>
Subject: Re: jbd2 inside a device mapper module
Date: Wed, 24 Dec 2008 20:38:18 -0200
Message-ID: <20081224223818.GY4127@blitiri.com.ar>
References: <20081224211038.GT4127@blitiri.com.ar>
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Cc: dm-devel@redhat.com, linux-ext4@vger.kernel.org
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[Adding lkml on the CC list, somehow I managed to screw the address and
sent it to the mm-cc list instead]

On Wed, Dec 24, 2008 at 07:10:38PM -0200, Alberto Bertogli wrote:
> 
> Hi!
> 
> I'm writing a small device mapper module, and I'm interested in placing
> a jbd/jdb2 journal on the backing device.
> 
> I started by trying to do a __bread() manually (just for early tests)
> inside my map function. But it got stucked, as far as I could see,
> waiting for a buffer head in wait_on_buffer() IIRC (I could track it
> down again if it's needed). And I couldn't find why it was locked, since
> it was an unused loopback device, and my code didn't even deal with
> buffer heads.
> 
> Then, since I was planning on using jbd/jdb2 anyway, I decided to use it
> (and went for jbd2).
> 
> 
> Now, I'm having issues with journal creation.
> 
> I tried using "mkfs.ext3 -O journal_dev", but jbd2_journal_load()
> complains that it can't find the journal superblock.
> 
> And if I modify jbd2_journal_create(), removing the 'if
> (journal->j_inode == NULL)' check (I imagine it's there for a reason,
> but from a quick look at the code couldn't find it and thought it was
> worth a try) then when creating it I get a warning (pasted below) and it
> gets locked up, which I think may be related to what happened when I did
> __bread(), but obviously I'm not sure at all.
> 
> And I got stucked there, so I thought it'd be better to ask. Does anyone
> have any ideas or suggestions on what I'm doing wrong?
> 
> 
> I've not published my code yet because it's really rough, but if anyone
> wants to take a look at it, please let me know. I was planning on
> posting it when it was at least working.
> 
> Thanks a lot,
> 		Alberto
> 
> 
> 
> [42949814.780000] ------------[ cut here ]------------
> [42949814.780000] WARNING: at /pub/src/linux/linux-2.6/fs/buffer.c:1186 mark_buffer_dirty+0x77/0xa0()
> [42949814.780000] Modules linked in:
> [42949814.780000] Call Trace:
> [42949814.780000] 678f17d8:  [<6003988b>] warn_on_slowpath+0x5b/0x80
> [42949814.780000] 678f1818:  [<600bbe9a>] __find_get_block_slow+0x7a/0x110
> [42949814.780000] 678f1858:  [<600bc219>] __find_get_block+0x79/0x180
> [42949814.780000] 678f1888:  [<60033b05>] __might_sleep+0x105/0x130
> [42949814.780000] 678f18c8:  [<600bc353>] __getblk+0x33/0x270
> [42949814.780000] 678f18f8:  [<600bc7e7>] mark_buffer_dirty+0x77/0xa0
> [42949814.780000] 678f1918:  [<6012b1a8>] jbd2_journal_create+0x88/0x170
> [42949814.780000] 678f1958:  [<601aac70>] csum_ctr+0x1b0/0x240
> [42949814.780000] 678f1968:  [<6019b810>] get_target_type+0x60/0xa0
> [42949814.780000] 678f19a8:  [<6019b0d4>] dm_table_add_target+0x174/0x3b0
> [42949814.780000] 678f1a08:  [<6019d057>] table_load+0xb7/0x200
> [42949814.780000] 678f1a68:  [<6019dd98>] dm_ctl_ioctl+0x288/0x300
> [42949814.780000] 678f1a98:  [<6019cfa0>] table_load+0x0/0x200
> [42949814.780000] 678f1c18:  [<600a82fb>] vfs_ioctl+0x1b/0x70
> [42949814.780000] 678f1c28:  [<600a8770>] do_vfs_ioctl+0x400/0x660
> [42949814.780000] 678f1ca8:  [<600a8a1a>] sys_ioctl+0x4a/0x80
> [42949814.780000] 678f1ce8:  [<6001a310>] handle_syscall+0x50/0x80
> [42949814.780000] 678f1d08:  [<6002bf1f>] userspace+0x3ff/0x530
> [42949814.780000] 678f1fc8:  [<60017012>] fork_handler+0x62/0x70
> [42949814.780000]
> [42949814.780000] ---[ end trace ebc125a00ee8f9d2 ]---