About leaf corruption recovery(currently only fs/subvol tree recovery)

[Qu Wenruo <quwenruo@cn.fujitsu.com>]

Hi all,

I'm trying to implement leaf corruption recovery.

*CURRENT BEHAVIOR*
Btrfs now heavily rely on chunk level duplication to protect its tree 
block(meta data).
That's completely good and works quite well.

However small device with mixed single chunk will suffer from the lack 
of duplication and when any
bit flip happens in tree block, the whole 16K leaf/node will be 
unreadable and finally cause
metadata corruption.

*OBJECT*
I hope btrfsck can repair such bit flip even with the cost of data lose.
(It will of course introduce data loss according to the following method)

And the ultimate object will be making a randomly slightly(0.2% of all 
bytes?) damaged btrfs
can pass btrfsck after repair.

*RECOVERY METHOD*
Current recovery method is consist of the following procedure:
1) find and record the unreadable extent buffers during normal fsck routine
With the record of the unreadable extent buffers, we can calculates the 
inode number range where
next step will drop.

2) *delete* the slot pointing to the leaf in parent node
Yes, delete the corrupted leaves, at least this is the cleanest and 
easiest method.
After the step, the metadata tree should at least be iteratable now.

3) cleanup the mess done in 2)
Need to do the following things in case btrfsck complains later
3.1) salvage data from extent tree in the deleting range.
Although fs/subvol leaf is deleted, extent data is still there, using 
EXTENT_ITEM in extent tree
may still recover some data.
Personally I prefer to create a lost+found dir in the root of its 
subvolume and use inode number as
file name to restore them.

3.2) Remove backref to the inodes in deleting ranges and move them if 
needed.
It is clear we need to remove the invalid backref, but if some inodes in 
deleting ranges casuing
its children files unaccessible from the subvolume root, then these 
files should be moved to 'lost+found' too,
even they are completely undamaged.

Although after the above steps, metadata like filename, access bits, 
owner, xattrs or inlined data will be
lost and some files/dirs will be moved to lost+found, it should at least 
btrfsck not complain any more.

*NEED ADVICE*
Any concern about the above recovery is welcomed, especially when some 
guy like me want to
implement such an aggressive recovery method.

Thanks
Qu