UBI: preserve corrupted PEBs

Currently UBI erases all corrupted eraseblocks, irrespectively of the nature
of corruption: corruption due to power cuts and non-power cut corruption.
The former case is OK, but the latter is not, because UBI may destroy
potentially important data.

With this patch, during scanning, when UBI hits a PEB with corrupted VID
header, it checks whether this PEB contains only 0xFF data. If yes, it is
safe to erase this PEB and it is put to the 'erase' list. If not, this may
be important data and it is better to avoid erasing this PEB. Instead,
UBI puts it to the corr list and moves out of the pool of available PEB.
IOW, UBI preserves this PEB.

Such corrupted PEB lessen the amount of available PEBs. So the more of them
we accumulate, the less PEBs are available. The maximum amount of non-power
cut corrupted PEBs is 8.

This patch is a response to UBIFS problem where reporter
(Matthew L. Creech <mlcreech@gmail.com>) observes that UBIFS index points
to an unmapped LEB. The theory is that corresponding PEB somehow got
corrupted and UBI wiped it. This patch (actually a series of patches)
tries to make sure such PEBs are preserved - this would make it is easier
to analyze the corruption.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

1 files changed, 19 insertions, 29 deletions

diff --git a/drivers/mtd/ubi/scan.c b/drivers/mtd/ubi/scan.c
index def0bf03d7fe..30b710216f26 100644
--- a/drivers/mtd/ubi/scan.c
+++ b/drivers/mtd/ubi/scan.c
@@ -706,8 +706,8 @@ out_free:
 struct ubi_scan_leb *ubi_scan_get_free_peb(struct ubi_device *ubi,
                                            struct ubi_scan_info *si)
 {
-        int err = 0, i;
-        struct ubi_scan_leb *seb;
+        int err = 0;
+        struct ubi_scan_leb *seb, *tmp_seb;
 
         if (!list_empty(&si->free)) {
                 seb = list_entry(si->free.next, struct ubi_scan_leb, u.list);
@@ -716,37 +716,27 @@ struct ubi_scan_leb *ubi_scan_get_free_peb(struct ubi_device *ubi,
                 return seb;
         }
 
-        for (i = 0; i < 2; i++) {
-                struct list_head *head;
-                struct ubi_scan_leb *tmp_seb;
-
-                if (i == 0)
-                        head = &si->erase;
-                else
-                        head = &si->corr;
-
-                /*
-                 * We try to erase the first physical eraseblock from the @head
-                 * list and pick it if we succeed, or try to erase the
-                 * next one if not. And so forth. We don't want to take care
-                 * about bad eraseblocks here - they'll be handled later.
-                 */
-                list_for_each_entry_safe(seb, tmp_seb, head, u.list) {
-                        if (seb->ec == UBI_SCAN_UNKNOWN_EC)
-                                seb->ec = si->mean_ec;
+        /*
+         * We try to erase the first physical eraseblock from the erase list
+         * and pick it if we succeed, or try to erase the next one if not. And
+         * so forth. We don't want to take care about bad eraseblocks here -
+         * they'll be handled later.
+         */
+        list_for_each_entry_safe(seb, tmp_seb, &si->erase, u.list) {
+                if (seb->ec == UBI_SCAN_UNKNOWN_EC)
+                        seb->ec = si->mean_ec;
 
-                        err = ubi_scan_erase_peb(ubi, si, seb->pnum, seb->ec+1);
-                        if (err)
-                                continue;
+                err = ubi_scan_erase_peb(ubi, si, seb->pnum, seb->ec+1);
+                if (err)
+                        continue;
 
-                        seb->ec += 1;
-                        list_del(&seb->u.list);
-                        dbg_bld("return PEB %d, EC %d", seb->pnum, seb->ec);
-                        return seb;
-                }
+                seb->ec += 1;
+                list_del(&seb->u.list);
+                dbg_bld("return PEB %d, EC %d", seb->pnum, seb->ec);
+                return seb;
         }
 
-        ubi_err("no eraseblocks found");
+        ubi_err("no free eraseblocks");
         return ERR_PTR(-ENOSPC);
 }