linux-ext4 - ext4 out of order when use cfq scheduler

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Message-ID: <697280a570654ae0aa1723fb7d11f51e@SGPMBX1004.APAC.bosch.com>
Date:	Tue, 22 Dec 2015 06:24:29 +0000
From:	"HUANG Weller (CM/EPF1-CN)" <Weller.Huang@...bosch.com>
To:	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
CC:	"Li, Michael" <huayil@....qualcomm.com>
Subject: ext4 out of order when use cfq scheduler

Hi EXT4 developers,

I am doing the power failure on a storage device. We are arm embedded system.


Test environment:
Kernel: 3.10.63
Test type: power failure
File create type: O_SYNC flag enabled
Mount option: data=ordered, journal_checksum,barrier=1
IO scheduler: CFQ

Debug method: kernel print every block number of write request. print whether it is journal block.  print whether it is (REQ_FUA | REQ_META | write)
After system startup, backup journal blocks before mount the test partition.

Below is analysis which I think there is a out of order:
hp0000017aMhWY3i0vMv is the file which checksum is error. Only half of the file contents is correct.


[root@...Fatlas6 ~]# debugfs /dev/nandblk0p3
debugfs 1.42.9 (28-Dec-2013)
debugfs:  imap test/hp0000017aMhWY3i0vMv
Inode 390 is part of block group 0
        located at block 141, offset 0x0280

00000280  80 81 00 00 10 00 04 00  c8 09 00 00 66 0a 00 00  |............f...|
00000290  66 0a 00 00 00 00 00 00  00 00 01 00 04 02 00 00  |f...............|
000002a0  00 00 08 00 01 00 00 00  0a f3 02 00 04 00 00 00  |................|
000002b0  00 00 00 00 00 00 00 00  80 00 00 00 00 2c 01 00  |.............,..|  ==> the file contents is at 0x00012c00
000002c0  80 00 00 00 01 00 00 00  8e 26 01 00 00 00 00 00  |.........&......|


Search the block number from the journal blocks:

[root@...Fatlas6 ~]# hexdump j.bin  -C |  grep "00 2c 01 00"
00039ab0  00 00 00 00 00 00 00 00  80 00 00 00 00 2c 01 00  |.............,..|

Search file name which the file checksum is error in journal blocks:

[root@...Fatlas6 ~]# hexdump j.bin  -C |  grep "3i0vMv" -B1
00030c60  86 01 00 00 1c 00 14 01  68 70 30 30 30 30 30 31  |........hp000001|
00030c70  37 61 4d 68 57 59 33 69  30 76 4d 76 88 01 00 00  |7aMhWY3i0vMv....|


List all journal block record to check which journal block records it:

[root@...Fatlas6 ~]# hexdump j.bin -C | grep "c0 3b 39 98"
00000000  c0 3b 39 98 00 00 00 04  00 00 00 00 00 00 08 00  |.;9.............|
00000800  c0 3b 39 98 00 00 00 05  00 00 00 6f 00 00 00 24  |.;9........o...$|
00001000  c0 3b 39 98 00 00 00 01  00 00 00 6f 00 00 00 75  |.;9........o...u|
0000c800  c0 3b 39 98 00 00 00 02  00 00 00 6f 00 00 00 00  |.;9........o....|
0000d000  c0 3b 39 98 00 00 00 01  00 00 00 70 00 00 00 65  |.;9........p...e|
00016000  c0 3b 39 98 00 00 00 02  00 00 00 70 00 00 00 00  |.;9........p....|
00016800  c0 3b 39 98 00 00 00 01  00 00 00 71 00 00 00 7c  |.;9........q...||
00021000  c0 3b 39 98 00 00 00 02  00 00 00 71 00 00 00 00  |.;9........q....|
00021800  c0 3b 39 98 00 00 00 01  00 00 00 72 00 00 00 82  |.;9........r....|
0002d000  c0 3b 39 98 00 00 00 02  00 00 00 72 00 00 00 00  |.;9........r....|
0002d800  c0 3b 39 98 00 00 00 01  00 00 00 73 00 00 00 88  |.;9........s....|  ==>00039ab0 is in last block, the file name and the start block number are all recorded in the journals.
0003a000  c0 3b 39 98 00 00 00 02  00 00 00 73 00 00 00 00  |.;9........s....|


Back to see the kernel log which it print all the block numbers:

...
...
[   46.222671] 244109  75277 
[   46.222693] 
[   46.272438] 244352  75520 
[   46.272460] 
[   46.348417] 238443  69611 
[   46.348438] 
[   46.349811] 244480  75648 
[   46.352287] 
[   46.404904] 244609  75777 
[   46.404926] 
[   46.454698] 244738  75906 
[   46.454719] 
[   46.505439] 244992  76160 
[   46.505459] 
[   46.557783] 245120  76288 
[   46.557804] 
[   46.610075] 245249  76417 
[   46.610096] 
[   46.660196] 245378  76546 
[   46.660219] 
[   46.709906] 201691  32859   ==> journal start is 32768, so the offset is 91, block size=2048,  so, the offset address in the j.bin is 0x2d800
[   46.709928] J [   46.711233] 
[   46.740635] drop to 9v
[   46.749540] 201716  32884 
[   46.749560] J S 
[   46.751039] 
[   46.753151] 245632  76800  ==> 76800 in hex is 0x012c00, it is the same start block of the file which checksum is error.
[   46.755284] 
nanddisk idle -> 1.
[   46.800227] 6v irq-2


The j.bin offset 0x2d800

0002d800  c0 3b 39 98 00 00 00 01  00 00 00 73 00 00 00 88  |.;9........s....|  ==>00039ab0 is in last block
0003a000  c0 3b 39 98 00 00 00 02  00 00 00 73 00 00 00 00  |.;9........s....|


Normally, ext4 will first write the file contents, and then write the journal and journal commit. Then after some delay, it will write the meta data. 
So The journal blocks contains the meta data of the file which the file contents already been written before.
But from above analysis,  the journal sequence 0x73 already contain the file name(hp0000017aMhWY3i0vMv) and the start block number(76800). So from the kernel log,  the block number(76800) should be available before the journal blocks but NOT after it.
It seems that there is out of order happen.

Could you please help to check this issue ? or give a explanation about it ?
Many thanks.

Best regards

Weller HUANG


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