UBIFS question: Power-cuts after ubifs_leb_unmap()

[Richard Weinberger <richard@nod.at>]

Artem, Adrian,

While playing with a new UBI/UBIFS test framework I managed to hit this error, 
with lprops self-checks enabled:

[ 2412.268964] UBIFS error (ubi0:0 pid 708): scan_check_cb: bad accounting of 
LEB 11: free 0, dirty 118072 flags 0x1, should be free 126976, dirty 0

LEB 11 is unmapped but LPT still thinks that some data is used,
lp->free + lp >dirty < leb-size.
Even without lprobs self-checks, the same filesystem will later hit this 
assertion in ubifs_garbage_collect_leb():

ubifs_assert(!list_empty(&sleb->nodes));

The assert makes sure that the LEB actually contains nodes.
ubifs_garbage_collect_leb() handles the special case lp->free + lp->dirty == 
c->leb_size.
But not lp->free + lp->dirty < leb-size.

Now I'm not sure where to fix that, maybe you can remember some design 
decisions.
1. Shall we massage ubifs_garbage_collect_leb() to deal with this special case 
too?
2. Is it already a bug when this case happens?

>From reviewing the code, I think the said situation can arise when we face 
power-cut
in ubifs_garbage_collect_leb():

if (snod->type == UBIFS_IDX_NODE) {
	...
} else {
	...

	err = ubifs_change_one_lp(c, lnum, c->leb_size, 0, 0, 0, 0);

	...

	err = ubifs_leb_unmap(c, lnum);

	// POWER CUT
}

We mark the LEB as free and unmap it.
ubifs_change_one_lp() does not immediately write a new LPT, if we lose power 
right after ubifs_leb_unmap() it can happen that the LEB already got erased 
but the LPT has the old accounting information.
UBIFS seems to expect such situations only for
lp->free + lp->dirty == c->leb_size

What do you think?

Thanks,
//richard