Date:	Fri, 7 Dec 2012 18:03:49 +0800
From:	Li Zefan <lizefan@...wei.com>
To:	Jan Kara <jack@...e.cz>
CC:	qixuan wu <wuqixuan@...il.com>, Tao Ma <tm@....ma>,
	"Theodore Ts'o" <tytso@....edu>, Eric Sandeen <sandeen@...hat.com>,
	Yafang Shao <laoar.shao@...il.com>,
	<linux-fsdevel@...r.kernel.org>, <linux-ext4@...r.kernel.org>,
	<wuqixuan@...wei.com>
Subject: Re: help about ext3 read-only issue on ext3(2.6.16.30)

On 2012/12/7 1:09, Jan Kara wrote:
> On Fri 07-12-12 00:21:25, qixuan wu wrote:
>> Hi Kara,
>>
>> On Thu, Dec 6, 2012 at 8:37 PM, Jan Kara <jack@...e.cz> wrote:
>>> On Thu 06-12-12 09:13:45, Li Zefan wrote:
>>>>>> I found this in one log:
>>>>>>
>>>>>> Nov 14 05:26:55 kernel: EXT3-fs error (device sda7): ext3_readdir: bad entry in directory #7225391: rec_len is smaller than minimal - offset=3952, inode=0, rec_len=0, name_len=0
>>>>>> Nov 14 13:42:40 kernel: EXT3-fs error (device sda7): ext3_readdir: bad entry in directory #7225391: rec_len is smaller than minimal - offset=4024, inode=0, rec_len=0, name_len=0
>>>>>> Nov 16 17:29:40 kernel: EXT3-fs error (device sda7): ext3_readdir: bad entry in directory #7225391: rec_len is smaller than minimal - offset=4084, inode=0, rec_len=0, name_len=0
>>>>>> Nov 23 19:42:44 kernel: EXT3-fs error (device sda7): ext3_readdir: bad entry in directory #7225391: rec_len is smaller than minimal - offset=3952, inode=0, rec_len=0, name_len=0
>>>   Sorry for posting here in the thread but I got unsubscribed from the
>>> list so I don't have the beginning of the thread in my inbox.
>>>
>>>   ext3 directory format is such that the last directory entry in the block
>>> should have length to exactly fill up the whole block. Apparently, the
>>> length got trimmed for some reason so we ended up before end of directory
>>> block looked of another directory entry there and didn't find anything. I
>>> will also make one observation regarding offsets. They are 3952, 4024, and
>>> 4084. If we subtract that from 4096 (block size), we get differences (in
>>> binary) 10010000, 01001000, 00001100. Interestingly these have always two
>>> bits set. Might be luck but need not...
>>
>> Yes, we also found the interesting things that the offset happen in
>> many boards are like below:
>> 1) 3952
>> 2) 3988( 3952+36)
>> 3) 4024( 3988+36)
>> 4) 4048(4042+24)
>> 5) 4084(same as the rec_len of ".." file if there isn't any file).
>>
>> I need introduce the rule of the files in the dir, for example:
>> .
>> ..
>> current_log.txt (len is 15, rec_len is 24 when there is file after it,
>> the value "24" i think has relative with  offset 4048)
>> 20120526124556.865213.txt(len is 25, rec_len is 36 when there is file after it).
>> 20120526124984.239475.txt(len is 25, rec_len is 36 when there is file after it).
>> ....
>> Because the rec_len is 36, it has some relative with those offset
>> values( the diff of those values are multiple of 36).
>> I need tell another thing, customer's app invoke opendir/readdir very
>> frequently. There are more than 1000 times, every second(the value
>> need to be confirmed).
>>
>>> Anyway it would be interesting to get the dump of the corrupted directory
>>> before e2fsck is run. You can do that by running:
>>>   debugfs -R "dump_inode <7225391> /tmp/corrupted_dir" /dev/sda7
>>>
>>> Then you can send the dump of the corrupted directory here.
>>
>> We have already dump of the data by debugfs. The data is very good
>> without error. But we just did it before fsck, even the fsck is not
>> giving any error. I want to know whether fsck will modify disk data
>> without reporting any error or not ?
>   Ah, OK. So it seems that directory block is OK, just  f_pos gets corrupted
> somehow. There are guards in ext3_readdir() to rescan dir block when
> directory is modified but maybe that's not working correctly. I don't want
> to burn too much time on this since this is so ancient kernel but I'd be
> looking in that direction...
> 

I've added some debug code into ext3, which does these things:
- dump the dir block
- print the current and last f_pos and offset
- dump_stack() to see which process triggers the bug

Hope we can trigger the bug in our labs (We did see this happened twice this week
in a lab), though we can't patch the kernel in the products.

I compared ext3_readdir() with latest ext3, and saw no difference except some
API changes. I'll dig deeper. Thansks for the suggestion!

Regards
Li Zefan

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