UBIFS: do not free write-buffers when in R/O mode

Currently UBIFS has a small optimization - it frees write-buffers when it is
re-mounted from R/W mode to R/O mode. Of course, when it is mounted R/O, it
does not allocate write-buffers as well.

This optimization is nice but it leads to subtle problems and complications
in recovery, which I can reproduce using the integck test. The symptoms are
that after a power cut the file-system cannot be mounted if we first mount
it R/O, and then re-mount R/W - 'ubifs_rcvry_gc_commit()' prints:

UBIFS error (pid 34456): could not find an empty LEB

Analysis of the  problem.

When mounting R/W, the reply process sets journal heads to buds [1], but
when mounting R/O - it does not do this, because the write-buffers are not
allocated. So 'ubifs_rcvry_gc_commit()' works completely differently for the
same file-system but for the following 2 cases:

1. mounting R/W after a power cut and recover
2. mounting R/O after a power cut, re-mounting R/W and run deferred recovery

In the former case, we have journal heads seeked to the a bud, in the latter
case, they are non-seeked (wbuf->lnum == -1). So in the latter case we do not
try to recover the GC LEB by garbage-collecting to the GC head, but we just
try to find an empty LEB, and there may be no empty LEBs, so we just fail.
On the other hand, in the former case (mount R/W), we are able to make a GC LEB
(@c->gc_lnum) by garbage-collecting.

Thus, let's remove this small nice optimization and always allocate
write-buffers. This should not make too big difference - we have only 3
of them, each of max. write unit size, which is usually 2KiB. So this is
about 6KiB of RAM for the typical case, and only when mounted R/O.

[1]: Note, currently the replay process is setting (seeking) the journal heads
to _some_ buds, not necessarily to the buds which had been the journal heads
before the power cut happened. This will be fixed separately.
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Cc: stable@kernel.org

fs/ubifs/log.c

@@ -174,26 +174,6 @@ void ubifs_add_bud(struct ubifs_info *c, struct ubifs_bud *bud)
 	spin_unlock(&c->buds_lock);
 }
 
-/**
- * ubifs_create_buds_lists - create journal head buds lists for remount rw.
- * @c: UBIFS file-system description object
- */
-void ubifs_create_buds_lists(struct ubifs_info *c)
-{
-	struct rb_node *p;
-
-	spin_lock(&c->buds_lock);
-	p = rb_first(&c->buds);
-	while (p) {
-		struct ubifs_bud *bud = rb_entry(p, struct ubifs_bud, rb);
-		struct ubifs_jhead *jhead = &c->jheads[bud->jhead];
-
-		list_add_tail(&bud->list, &jhead->buds_list);
-		p = rb_next(p);
-	}
-	spin_unlock(&c->buds_lock);
-}
-
 /**
  * ubifs_add_bud_to_log - add a new bud to the log.
  * @c: UBIFS file-system description object

fs/ubifs/super.c

@@ -1257,12 +1257,12 @@ static int mount_ubifs(struct ubifs_info *c)
 		goto out_free;
 	}
 
+	err = alloc_wbufs(c);
+	if (err)
+		goto out_cbuf;
+
 	sprintf(c->bgt_name, BGT_NAME_PATTERN, c->vi.ubi_num, c->vi.vol_id);
 	if (!c->ro_mount) {
-		err = alloc_wbufs(c);
-		if (err)
-			goto out_cbuf;
-
 		/* Create background thread */
 		c->bgt = kthread_create(ubifs_bg_thread, c, "%s", c->bgt_name);
 		if (IS_ERR(c->bgt)) {
@@ -1631,12 +1631,6 @@ static int ubifs_remount_rw(struct ubifs_info *c)
 	if (err)
 		goto out;
 
-	err = alloc_wbufs(c);
-	if (err)
-		goto out;
-
-	ubifs_create_buds_lists(c);
-
 	/* Create background thread */
 	c->bgt = kthread_create(ubifs_bg_thread, c, "%s", c->bgt_name);
 	if (IS_ERR(c->bgt)) {
@@ -1744,7 +1738,6 @@ static void ubifs_remount_ro(struct ubifs_info *c)
 	if (err)
 		ubifs_ro_mode(c, err);
 
-	free_wbufs(c);
 	vfree(c->orph_buf);
 	c->orph_buf = NULL;
 	kfree(c->write_reserve_buf);