ubi: why ubi wear leveling always pick highly worn-out free physical eraseblock as the mov_to target

[Zhihao Cheng]

在 2024/2/23 11:27, Ryder Wang 写道:
> Refer to the ubi source code:
> wear_leveling_worker
>    e2 = get_peb_for_wl(ubi)
>      e = find_wl_entry(ubi, &ubi->free, WL_FREE_MAX_DIFF);
>        
> The function find_wl_entry() always find the  highly worn-out free physical eraseblock (e2):
> 1. It's good to check such PEB (e2) to decide whether the following wear leveling procedure should be continued according to UBI_WL_THRESHOLD.
> 2. But personally I can't understand why such high worn-out free physical eraseblock should also be used as target PEB(move_to) to store the move_from data for wear leveling purpose. Will it be much more reasonable to use low worn-out free physical eraseblock (from ubi->free tree) in this case for more perfect wear leveling?.
> 

Normally, e1 is the smaller ec counter picked from ubi->used, e2 is 
bigger ec counter picked from ubi->free, the wear-leveling worker 
follows that rule which is based on the realization of free PEBs fetching.
First, let's talk about the simplest case without fastmap, assumpt that 
CONFIG_MTD_UBI_WL_THRESHOLD=2, there are total 3 PEBs.
Only 1 EB(erase block) is used, then free it, and repeat the process 
forvever. According to the implementation in 
ubi_wl_get_peb->wl_get_wle->find_mean_wl_entry, the ubi->free tree will 
change like:
(1,1,1) -> (1,1,2) -> (1,2,2) -> (1,2,3) -> (1,3,3) -> ... -> (1,4,5) -> 
(2,4,5), which means that wear-leveling worker is not needed, because 
find_mean_wl_entry has made sure that 'max_ec - min_ec <= 2 * 
CONFIG_MTD_UBI_WL_THRESHOLD'. Similar, when there are more EBs are used 
and freed, there is no need to trigger wear-leveling work.
However, if one free PEB is taken and not be freed, other free PEBs are 
taken and freed, like:
ubi->used: 1
ubi->free: (1, 1) -> (1, 2) -> ... (x, y)
After a while, min(x,y) - 1 will be greater than 
2*CONFIG_MTD_UBI_WL_THRESHOLD. Then, wear-leveling should start working, 
cold data always takes certain PEB for a long time, which causes PEBs 
with smaller ec counter gather in ubi->used and PEBs with bigger ec 
counter gather in ubi->freed.
I think that's the explainations of the rule how to pick e1/e2.