UBIFS: recovery of master node

[Andrea Scian <rnd4@dave-tech.it>]

Dear all,

I'm trying to understand how UBIFS recovers master node from a corrupted 
flash (e.g. after power cut during one of the two of master node or due 
flash corruption itself).

IIUC (please correct me if I'm wrong) UBIFS store two copies of master 
node, in LEB1 and LEB2 (LEB0 is reserved for superblock), ref. 
http://www.linux-mtd.infradead.org/doc/ubifs_whitepaper.pdf.
Inside ubifs_recover_master_node() try to read first LEB1 and, only in 
case of get_master_node() does NOT return an error, it tries to read 
LEB2 (ref. fs/ubifs/recovery.c)
I'm working with a 3.10 class kernel, but I've found nearly the same 
code on mainline.

On my test-bed, the recovery fails because get_master_node(LEB1) fails 
(exactly here 
http://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/tree/fs/ubifs/recovery.c#n184)
I'm hacking around the code to see what's really happening under the 
wood, but I'm a UBIFS newbie I would like to ask:

1) why if get_master_node(LEB1) fails we don't ALWAYS look at 
get_master_node(LEB2)? I think we should try to read LEB2 even if 
something really bad happens to LEB1.. or not?

2) if I bypass the get_master_node(LEB1) return value, I found that 
get_master_node(LEB2) fails too, for the same reason of LEB1 (see 
above). IIUC we check about empty space because master node pages get 
written without being erased every time, but I'm still studying this 
topic ;-)

If I bypass that check too, I can mount UBIFS and everything inside the 
FS is there but, of course, I'm sure I'm doing something that may be wrong..

WDYT?

Thanks in advance and kind regards,

-- 

Andrea SCIAN

DAVE Embedded Systems