UBI: badblock management confusion!

[Nancy]

Hi all,
        I don't quite understand the UBI badblock management unit.
But I think there must be something wrong, or I take a wrong operation.
        Please follow my test below:

# ubiformat /dev/mtd5
ubiformat: mtd5 (NAND), size 536870912 bytes (512.0 MiB), 262144 eraseblocks of
262144 bytes (256.0 KiB), min. I/O size 2048 bytes
libscan: scanning eraseblock 2047 -- 100 % complete
ubiformat: 2045 eraseblocks have valid erase counter, mean value is 23
ubiformat: bad eraseblocks: 10, 40, 84
ubiformat: formatting eraseblock 2047 -- 100 % complete

# modprobe ubi mtd=5
UBI: physical eraseblock size:   262144 bytes (256 KiB)
UBI: logical eraseblock size:    258048 bytes
UBI: smallest flash I/O unit:    2048
UBI: VID header offset:          2048 (aligned 2048)
UBI: data offset:                4096
UBI: attached mtd5 to ubi0
UBI: MTD device name:            "NAND VFAT partition"
UBI: MTD device size:            512 MiB
UBI: number of good PEBs:        2045
UBI: number of bad PEBs:         3
UBI: max. allowed volumes:       128
UBI: wear-leveling threshold:    256
UBI: number of internal volumes: 1
UBI: number of user volumes:     0
UBI: available PEBs:             2021
UBI: total number of reserved PEBs: 24
UBI: number of PEBs reserved for bad PEB handling: 20
UBI: max/mean erase counter: 38/24
UBI: background thread "ubi_bgt0d" started, PID 233

***************************************************************************
I'm going to make one big volume which take all available space this ubi
device has.

I think the volume size should be equal to
(total number of PEB this ubi device has - total number of reserved PEBs) \
* LEB size
In this case, it should be : (2048-24) * 258048 bytes

But that is not what UBI count.
UBI think the volume size
= (number of good PEBs - total number of reserved PEBs) * LEB size

Q: Number of bad PEBs at initialization are not included in resered for bad
PEBs handling. That mean there should never has badblocks overflow issue
happen. What a good idea! But is there really a possible way to implement this
idea?
   Suppose there's only one volume take all the ubi device space, there's no
badblocks at the very beginning. The user does never shut down the device but
hibernate for a long time. Then, suppose there is 3 badblocks and the volume
just full of data. No space left. Then reboot the device, beside the 3 badblocks
which used to be count in reserved PEBs, now be seperate from the reserved PEBs,
But there's no space, reserved PEB will fail.

   The following test showes even the volume data is blank, there still comes
problem.
   Is it an UBI bug or my wrong operation?
   Thanks for your notice about it!
****************************************************************************

# ubimkvol /dev/ubi0 -n 0 -N vfat -s 509292KiB
Volume ID 0, size 2021 LEBs (521515008 bytes, 497.4 MiB), LEB size
258048 bytes (252.0 KiB), dynamic, name "vfat", alignment 1
# rmmod ubi
UBI: mtd5 is detached from ubi0

*****************************************************************************
"markbadblock" is a tool I force to mark a block bad manually, just for test
*****************************************************************************
# markbadblock -l 17 /dev/mtd5
Block size 262144, page size 2048, OOB size 64
mark block 17 to be badblock done

# modprobe ubi mtd=5
UBI: physical eraseblock size:   262144 bytes (256 KiB)
UBI: logical eraseblock size:    258048 bytes
UBI: smallest flash I/O unit:    2048
UBI: VID header offset:          2048 (aligned 2048)
UBI: data offset:                4096
UBI warning: ubi_eba_init_scan: cannot reserve enough PEBs for bad PEB
handling, reserved 19, need 20
UBI: attached mtd5 to ubi0
UBI: MTD device name:            "NAND VFAT partition"
UBI: MTD device size:            512 MiB
UBI: number of good PEBs:        2044
UBI: number of bad PEBs:         4
UBI: max. allowed volumes:       128
UBI: wear-leveling threshold:    256
UBI: number of internal volumes: 1
UBI: number of user volumes:     1
UBI: available PEBs:             0
UBI: total number of reserved PEBs: 2044
UBI: number of PEBs reserved for bad PEB handling: 19
UBI: max/mean erase counter: 39/24
UBI: background thread "ubi_bgt0d" started, PID 263
# modprobe ubifs

# mount -t ubifs ubi0:ubifs /mnt/1
UBIFS error (pid 293): ubifs_get_sb: cannot open "ubi0:ubifs", error -19
mount: mounting ubi0:ubifs on /mnt/1 failed: No such device
# rmmod ubifs
# rmmod ubi

# ubiformat /dev/mtd5
ubiformat: mtd5 (NAND), size 536870912 bytes (512.0 MiB), 262144 eraseblocks of
262144 bytes (256.0 KiB), min. I/O size 2048 bytes
libscan: scanning eraseblock 2047 -- 100 % complete
ubiformat: 2044 eraseblocks have valid erase counter, mean value is 24
ubiformat: bad eraseblocks: 10, 17, 40, 84
ubiformat: formatting eraseblock 2047 -- 100 % complete

-- 
Best wishes,
Nancy