UBI: cleanup comments about corrupted PEBs
Just make them a bit more readable and explanatory. Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
This commit is contained in:
parent
6c1e875ca6
commit
fef2deb31f
1 changed files with 34 additions and 20 deletions
|
@ -39,32 +39,46 @@
|
|||
* eraseblocks are put to the @free list and the physical eraseblock to be
|
||||
* erased are put to the @erase list.
|
||||
*
|
||||
* About corruptions
|
||||
* ~~~~~~~~~~~~~~~~~
|
||||
*
|
||||
* UBI protects EC and VID headers with CRC-32 checksums, so it can detect
|
||||
* whether the headers are corrupted or not. Sometimes UBI also protects the
|
||||
* data with CRC-32, e.g., when it executes the atomic LEB change operation, or
|
||||
* when it moves the contents of a PEB for wear-leveling purposes.
|
||||
*
|
||||
* UBI tries to distinguish between 2 types of corruptions.
|
||||
* 1. Corruptions caused by power cuts. These are harmless and expected
|
||||
* corruptions and UBI tries to handle them gracefully, without printing too
|
||||
* many warnings and error messages. The idea is that we do not lose
|
||||
* important data in these case - we may lose only the data which was being
|
||||
* written to the media just before the power cut happened, and the upper
|
||||
* layers (e.g., UBIFS) are supposed to handle these situations. UBI puts
|
||||
* these PEBs to the head of the @erase list and they are scheduled for
|
||||
* erasure.
|
||||
*
|
||||
* 1. Corruptions caused by power cuts. These are expected corruptions and UBI
|
||||
* tries to handle them gracefully, without printing too many warnings and
|
||||
* error messages. The idea is that we do not lose important data in these case
|
||||
* - we may lose only the data which was being written to the media just before
|
||||
* the power cut happened, and the upper layers (e.g., UBIFS) are supposed to
|
||||
* handle such data losses (e.g., by using the FS journal).
|
||||
*
|
||||
* When UBI detects a corruption (CRC-32 mismatch) in a PEB, and it looks like
|
||||
* the reason is a power cut, UBI puts this PEB to the @erase list, and all
|
||||
* PEBs in the @erase list are scheduled for erasure later.
|
||||
*
|
||||
* 2. Unexpected corruptions which are not caused by power cuts. During
|
||||
* scanning, such PEBs are put to the @corr list and UBI preserves them.
|
||||
* Obviously, this lessens the amount of available PEBs, and if at some
|
||||
* point UBI runs out of free PEBs, it switches to R/O mode. UBI also loudly
|
||||
* informs about such PEBs every time the MTD device is attached.
|
||||
* scanning, such PEBs are put to the @corr list and UBI preserves them.
|
||||
* Obviously, this lessens the amount of available PEBs, and if at some point
|
||||
* UBI runs out of free PEBs, it switches to R/O mode. UBI also loudly informs
|
||||
* about such PEBs every time the MTD device is attached.
|
||||
*
|
||||
* However, it is difficult to reliably distinguish between these types of
|
||||
* corruptions and UBI's strategy is as follows. UBI assumes (2.) if the VID
|
||||
* header is corrupted and the data area does not contain all 0xFFs, and there
|
||||
* were not bit-flips or integrity errors while reading the data area. Otherwise
|
||||
* UBI assumes (1.). The assumptions are:
|
||||
* o if the data area contains only 0xFFs, there is no data, and it is safe
|
||||
* to just erase this PEB.
|
||||
* o if the data area has bit-flips and data integrity errors (ECC errors on
|
||||
* corruptions and UBI's strategy is as follows. UBI assumes corruption type 2
|
||||
* if the VID header is corrupted and the data area does not contain all 0xFFs,
|
||||
* and there were no bit-flips or integrity errors while reading the data area.
|
||||
* Otherwise UBI assumes corruption type 1. So the decision criteria are as
|
||||
* follows.
|
||||
* o If the data area contains only 0xFFs, there is no data, and it is safe
|
||||
* to just erase this PEB - this is corruption type 1.
|
||||
* o If the data area has bit-flips or data integrity errors (ECC errors on
|
||||
* NAND), it is probably a PEB which was being erased when power cut
|
||||
* happened.
|
||||
* happened, so this is corruption type 1. However, this is just a guess,
|
||||
* which might be wrong.
|
||||
* o Otherwise this it corruption type 2.
|
||||
*/
|
||||
|
||||
#include <linux/err.h>
|
||||
|
|
Reference in a new issue