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Commit be7b42a5 authored by Artem Bityutskiy's avatar Artem Bityutskiy
Browse files

UBIFS: describe UBIFS recovery logic some more 



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

Signed-off-by: default avatarArtem Bityutskiy <Artem.Bityutskiy@nokia.com>
parent 822ed64c

fs/ubifs/recovery.c

+21 −0
Original line number Diff line number Diff line
@@ -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 */