Re: [RFC PATCH] UBI fixable bit-flip issue

[Boris Brezillon <boris.brezillon@xxxxxxxxxxx>]

Hi Mark,

On Fri, 17 Aug 2018 10:34:21 +1000
Mark Spieth <mspieth@xxxxxxxxxxxxxxxxx> wrote:

> Hi
> 
> Richard Weinberger suggested I post this here. It is also in the uboot 
> mailing list
> 
> In the process of investigating a boot failure on one of our devices, the
> 
> UBI: fixable bit-flip detected at PEB
> 
> message was seen with the following behaviour during kernel load in u-boot.
> 
> Read [2285568] bytes
> UBI: fixable bit-flip detected at PEB 415
> UBI: schedule PEB 415 for scrubbing
> UBI: fixable bit-flip detected at PEB 415
> UBI: fixable bit-flip detected at PEB 419
> UBI: schedule PEB 419 for scrubbing
> UBI: fixable bit-flip detected at PEB 419
> UBI: fixable bit-flip detected at PEB 420
> UBI: schedule PEB 420 for scrubbing
> UBI: fixable bit-flip detected at PEB 420
> UBI: fixable bit-flip detected at PEB 419
> UBI: fixable bit-flip detected at PEB 420
> UBI: fixable bit-flip detected at PEB 419
> UBI: fixable bit-flip detected at PEB 420
> UBI: fixable bit-flip detected at PEB 419
> UBI: fixable bit-flip detected at PEB 420
> UBI: fixable bit-flip detected at PEB 419
> UBI: fixable bit-flip detected at PEB 420
> UBI: fixable bit-flip detected at PEB 419
> UBI: fixable bit-flip detected at PEB 420
> UBI: fixable bit-flip detected at PEB 419
> 
> his repeats until reset.
> 
> U boot is a patched version of 2010.06 supplied by the chip vendor. No 
> newer version is available from the vendor to try.
> 
> The patches include the init eba/wl swap.
> 
> A more detailed log with debugging available follows:
> 
> UBI: fixable bit-flip detected at PEB 419
> UBI DBG: schedule_erase: schedule erasure of PEB 419, EC 19, torture 0
> UBI DBG: erase_worker: erase PEB 419 EC 19
> UBI DBG: sync_erase: erase PEB 419, old EC 19
> UBI DBG: do_sync_erase: erase PEB 419
> UBI DBG: sync_erase: erased PEB 419, new EC 20
> UBI DBG: ubi_io_write_ec_hdr: write EC header to PEB 419
> UBI DBG: ubi_io_write: write 2048 bytes to PEB 419:0
> UBI DBG: ensure_wear_leveling: schedule scrubbing
> UBI DBG: wear_leveling_worker: scrub PEB 420 to PEB 419
> UBI DBG: ubi_io_read_vid_hdr: read VID header from PEB 420
> UBI DBG: ubi_io_read: read 2048 bytes from PEB 420:2048
> UBI DBG: ubi_eba_copy_leb: copy LEB 6:11, PEB 420 to PEB 419
> UBI DBG: ubi_eba_copy_leb: read 126976 bytes of data
> UBI DBG: ubi_io_read: read 126976 bytes from PEB 420:4096
> UBI: fixable bit-flip detected at PEB 420
> UBI DBG: ubi_io_write_vid_hdr: write VID header to PEB 419
> UBI DBG: ubi_io_write: write 2048 bytes to PEB 419:2048
> UBI DBG: ubi_io_read_vid_hdr: read VID header from PEB 419
> UBI DBG: ubi_io_read: read 2048 bytes from PEB 419:2048
> UBI DBG: ubi_io_write: write 126976 bytes to PEB 419:4096
> UBI DBG: ubi_io_read: read 126976 bytes from PEB 419:4096
> UBI: fixable bit-flip detected at PEB 419
> UBI DBG: schedule_erase: schedule erasure of PEB 419, EC 20, torture 0
> UBI DBG: erase_worker: erase PEB 419 EC 20
> UBI DBG: sync_erase: erase PEB 419, old EC 20
> UBI DBG: do_sync_erase: erase PEB 419
> UBI DBG: sync_erase: erased PEB 419, new EC 21
> UBI DBG: ubi_io_write_ec_hdr: write EC header to PEB 419
> UBI DBG: ubi_io_write: write 2048 bytes to PEB 419:0
> UBI DBG: ensure_wear_leveling: schedule scrubbing
> UBI DBG: wear_leveling_worker: scrub PEB 420 to PEB 419
> UBI DBG: ubi_io_read_vid_hdr: read VID header from PEB 420
> UBI DBG: ubi_io_read: read 2048 bytes from PEB 420:2048
> UBI DBG: ubi_eba_copy_leb: copy LEB 6:11, PEB 420 to PEB 419
> UBI DBG: ubi_eba_copy_leb: read 126976 bytes of data
> UBI DBG: ubi_io_read: read 126976 bytes from PEB 420:4096
> UBI: fixable bit-flip detected at PEB 420
> UBI DBG: ubi_io_write_vid_hdr: write VID header to PEB 419
> UBI DBG: ubi_io_write: write 2048 bytes to PEB 419:2048
> UBI DBG: ubi_io_read_vid_hdr: read VID header from PEB 419
> UBI DBG: ubi_io_read: read 2048 bytes from PEB 419:2048
> UBI DBG: ubi_io_write: write 126976 bytes to PEB 419:4096
> UBI DBG: ubi_io_read: read 126976 bytes from PEB 419:4096
> UBI: fixable bit-flip detected at PEB 419
> 
> Investigation showed that a read with correctable bit errors was done 
> returning -EUCLEAN to the ubi read function.
> 
> Having read 
> https://lists.denx.de/pipermail/u-boot/2013-September/161961.html which 
> details a workaround to not return EUCLEAN from the NAND reader unless 
> the number of fixed bits returned was 75% of the total number of 
> correctable bits was exceeded during the read. This was impleneted in 
> this version of ubi in uboot 2010.06 and it does hide the bit-flip 
> infinite issue since this is new NAND FLASH. The original 2010.06 
> implementation returns EUCLEAN for any number of fixable bit flips and 
> thus causes the PEB move to the best free one (scrub mode in 
> wear_leveling_worker).

What's your NAND ECC requirements, and how many bitflips do you
actually have in those blocks? Also, which NAND controller are we
talking about?

> 
> This fix is not a root cause fix though. Investigating further led to 
> the following root cause solution. The following is AFAICT.
> 
> When the scrubber chooses a PEB to move the from the free balanced tree. 
> This tree is sorted by EC (erase count) and then by PEB number.
> 
> The find_wl_entry call uses a max parameter of WL_FREE_MAX_DIFF which is 
> 8192 in this config. So the find_wl_entry function will find a PEB that 
> is better in erase count that the current PEB EC. This can easily cause 
> it to find the PEB that was just moved from if it is the lowest numbered 
> PEB in the free tree. Waiting for EC to go above 8192 would take a long 
> time and cause premature aging of the flash PEBs in question.
> 
> The easy solution is to change the max parameter for scrubbing to this 
> call to 0 so it finds a PEB with a smaller EC than the one being 
> replaced. This means it wont use the previously discarded PEB as its 
> first choice.

Setting it to 0 sounds a bit aggressive. I guess the idea behind this
MAX_DIFF was to avoid spending too much time searching for the smallest
EC val when most of them are close enough. On the other hand, 8192 is
big an probably only suitable for NANDs that allows 100000 PE cycles.

> 
> This fix was implemented and fixable bit-flip errors no longer 
> hang/freeze the boot process! UBI erase and reformat was used between 
> re-tests to get consistent results.
> 
> Adding the above 75% correctable bitflip threshold is also a good thing 
> as less movement will ensue when the FLASH is new, but as the flash 
> ages, the root cause will once again be invoked causing un-recoverable 
> boot failures.

It shouldn't. As long as you configure the threshold to a proper
value (if you think 75% is too high, set it to 50%) UBI should have
time to detect blocks containing too many bitflips and move them
around.

> 
> Note this fault is also in the latest kernel drivers for UBI and may 
> also exist in other wear leveling implementations. The kernel driver 
> issue may be at fault for android devices locking up/freezing 
> sporadically during FLASH read when scrubbing with a relatively full 
> flash and marginally correctable errors causing ping pong PEB moves.
> 
> The following patch is a workaround and is almost certainly not an 
> optimal solution.
> 
> What is required for CONFIG_MTD_UBI_FASTMAP is uncertain.
> 
> I am in the process of writing a unit test to highlight this ping ping 
> move behaviour but have not completed that yet.
> 
> I hope this description is clear enough.

Well, I think selecting the bitflip threshold properly is really
important, simply because some NANDs (including SLC NANDs) are showing
bitflips even on blocks that have a low EC. Check the NAND ECC
requirements, and if it's something like 8bit/512bytes, I guess that's
more or less expected (it all depends on how many bitflips you have in
the faulty block). It's less likely on NANDs requiring 1bit/512bytes
ECC, and if that happens on such NANDs, you may have a problem in the
controller driver.

Regards,

Boris

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