UBIFS Index Node Corruption - Invalid Key Type

[DHANAPAL, GNANACHANDRAN (G.)]

Hi There,

We have i.MX6d based platform that has FSL BSP 3.10.17 running. 
There is SLC Micron NAND of size 2GB with 512K Block size and 4K min I/O  used
for software storage. This NAND has got 7 partitions, Among that two partitions
have Read-Write ubifs file system for runtime data and one has Read-only ubifs 
file system that has all the system files. Recently we have observed a issue that 
one of RW ubifs partition failed to mount due to invalid key type in one of index 
node in ubifs. we have seen the same issue in two units out of 1000 units running 
in the field.

Following is the index node that has  invalid key in its first branch.
key type = 0xa917dd4c >> 29 = 5 (invalid key)

        magic          0x6101831
	crc            0x944add59
	node_type      9 (indexing node)
	group_type     0 (no node group)
	sqnum          378678
	len            148
	child_cnt      6
	level          2
	Branches:
	0: LEB 569:280144 len 108 key (bad key type: 0x000001, 0xa917dd4c)
	1: LEB 446:349792 len 108 key (5595, inode)
	2: LEB 584:361528 len 108 key (5822, inode)
	3: LEB 602:255360 len 108 key (6050, inode)
	4: LEB 613:480280 len 188 key (6258, inode)
	5: LEB 614:373784 len 88 key (6521, data, 25)

Please help us to understand following points

	1. Did this happen because abrupt power cut ? if so, how could CRC of the node still be intact?
	2. Could there be any bit flip ? but still again CRC intact.
	3. Would the key pointed by  corrupted branch be  UBIFS_DENT_KEY or  UBIFS_XENT_KEY ? 
		Because  29 bit of key[0] (0xa917dd4c  & 0x1FFFFFFF) seems to have some kind of  hash value.
		but node (at leb 569:280144)  that is pointed by corrupted branch seem to be index node.
	4. This corrupted entry is always seen  in first branch of an index node  in two failed units. Could there be reason?
	5. Since CRC is intact, would there be any possibility or scenario that corrupted node can be written with valid CRC?


Please share us if you have any suggestions or comments.

Attachement: Debug mount log of this partition. 
	      mtd Partition : /dev/mtd7
	      ubifs partition name : meta

Cheers
Gnana


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