UBIFS: recovery of master node
Andrea Scian
rnd4 at dave-tech.it
Thu Jul 16 06:22:12 PDT 2015
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
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