UBIFS: fix debugging failure in dbg_check_space_info

This patch fixes a debugging failure with which looks like this:
UBIFS error (pid 32313): dbg_check_space_info: free space changed from 6019344 to 6022654

The reason for this failure is described in the comment this patch adds
to the code. But in short - 'c->freeable_cnt' may be different before
and after re-mounting, and this is normal. So the debugging code should
make sure that free space calculations do not depend on 'c->freeable_cnt'.

A similar issue has been reported here:
http://lists.infradead.org/pipermail/linux-mtd/2011-April/034647.html

This patch should fix it.

For the -stable guys: this patch is only relevant for kernels 2.6.30
onwards.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Cc: stable@kernel.org [2.6.30+]

1 files changed, 36 insertions, 5 deletions

diff --git a/fs/ubifs/debug.c b/fs/ubifs/debug.c
index 645737769c98..004d3745dc45 100644
--- a/fs/ubifs/debug.c
+++ b/fs/ubifs/debug.c
@@ -972,11 +972,39 @@ void dbg_dump_index(struct ubifs_info *c)
 void dbg_save_space_info(struct ubifs_info *c)
 {
         struct ubifs_debug_info *d = c->dbg;
-
-        ubifs_get_lp_stats(c, &d->saved_lst);
+        int freeable_cnt;
 
         spin_lock(&c->space_lock);
+        memcpy(&d->saved_lst, &c->lst, sizeof(struct ubifs_lp_stats));
+
+        /*
+         * We use a dirty hack here and zero out @c->freeable_cnt, because it
+         * affects the free space calculations, and UBIFS might not know about
+         * all freeable eraseblocks. Indeed, we know about freeable eraseblocks
+         * only when we read their lprops, and we do this only lazily, upon the
+         * need. So at any given point of time @c->freeable_cnt might be not
+         * exactly accurate.
+         *
+         * Just one example about the issue we hit when we did not zero
+         * @c->freeable_cnt.
+         * 1. The file-system is mounted R/O, c->freeable_cnt is %0. We save the
+         *    amount of free space in @d->saved_free
+         * 2. We re-mount R/W, which makes UBIFS to read the "lsave"
+         *    information from flash, where we cache LEBs from various
+         *    categories ('ubifs_remount_fs()' -> 'ubifs_lpt_init()'
+         *    -> 'lpt_init_wr()' -> 'read_lsave()' -> 'ubifs_lpt_lookup()'
+         *    -> 'ubifs_get_pnode()' -> 'update_cats()'
+         *    -> 'ubifs_add_to_cat()').
+         * 3. Lsave contains a freeable eraseblock, and @c->freeable_cnt
+         *    becomes %1.
+         * 4. We calculate the amount of free space when the re-mount is
+         *    finished in 'dbg_check_space_info()' and it does not match
+         *    @d->saved_free.
+         */
+        freeable_cnt = c->freeable_cnt;
+        c->freeable_cnt = 0;
         d->saved_free = ubifs_get_free_space_nolock(c);
+        c->freeable_cnt = freeable_cnt;
         spin_unlock(&c->space_lock);
 }
 
@@ -993,12 +1021,15 @@ int dbg_check_space_info(struct ubifs_info *c)
 {
         struct ubifs_debug_info *d = c->dbg;
         struct ubifs_lp_stats lst;
-        long long avail, free;
+        long long free;
+        int freeable_cnt;
 
         spin_lock(&c->space_lock);
-        avail = ubifs_calc_available(c, c->min_idx_lebs);
+        freeable_cnt = c->freeable_cnt;
+        c->freeable_cnt = 0;
+        free = ubifs_get_free_space_nolock(c);
+        c->freeable_cnt = freeable_cnt;
         spin_unlock(&c->space_lock);
-        free = ubifs_get_free_space(c);
 
         if (free != d->saved_free) {
                 ubifs_err("free space changed from %lld to %lld",