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UBIFS: describe UBIFS recovery logic some more

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This patch adds more commentaries about UBIFS recovery logic which should
explain the famous UBIFS "corrupt empty space" errors.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
This commit is contained in:
Artem Bityutskiy 2011-02-06 16:41:06 +02:00
parent 822ed64c5b
commit be7b42a5cb
1 changed files with 21 additions and 0 deletions
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21
fs/ubifs/recovery.c
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@ -28,6 +28,23 @@
* UBIFS always cleans away all remnants of an unclean un-mount, so that
* errors do not accumulate. However UBIFS defers recovery if it is mounted
* read-only, and the flash is not modified in that case.
*
* The general UBIFS approach to the recovery is that it recovers from
* corruptions which could be caused by power cuts, but it refuses to recover
* from corruption caused by other reasons. And UBIFS tries to distinguish
* between these 2 reasons of corruptions and silently recover in the former
* case and loudly complain in the latter case.
*
* UBIFS writes only to erased LEBs, so it writes only to the flash space
* containing only 0xFFs. UBIFS also always writes strictly from the beginning
* of the LEB to the end. And UBIFS assumes that the underlying flash media
* writes in @c->min_io_unit bytes at a time.
*
* Hence, if UBIFS finds a corrupted node at offset X, it expects only the min.
* I/O unit corresponding to offset X to contain corrupted data, all the
* following min. I/O units have to contain empty space (all 0xFFs). If this is
* not true, the corruption cannot be the result of a power cut, and UBIFS
* refuses to mount.
*/
#include <linux/crc32.h>
@ -671,6 +688,10 @@ struct ubifs_scan_leb *ubifs_recover_leb(struct ubifs_info *c, int lnum,
} else {
int corruption = first_non_ff(buf, len);
/*
* See header comment for this file for more
* explanations about the reasons we have this check.
*/
ubifs_err("corrupt empty space LEB %d:%d, corruption "
"starts at %d", lnum, offs, corruption);
/* Make sure we dump interesting non-0xFF data */