Re: Does UBI LEB-level access interlock happily with UBIfs access?

[Artem Bityutskiy <dedekind1@gmail.com>]

On Tue, 2014-09-16 at 10:04 -0400, Atlant Schmidt wrote:
> Folks:
> 
>   We use the ordinary MTD/UBI/UBIfs stack on our
>   Embedded Linux system.
> 
>   For the purposes of scrubbing-out single bit errors,
>   I'd like to read through all of the LEBs stored in the
>   UBI device and whenever the ECC information indicates
>   that any correctable errors occurred, I'd like to
>   *RE-WRITE* that LEB (thereby forcing it to be scrubbed).
>   (Note: I might do this page-by-page rather than LEB-
>   by-LEB.)

Well, you may do something like

dd if=/dev/ubiX_Y of=/dev/null

For all volumes, and this will make UBI real all pages from all volumes
and whenever there was a bit-flip, schedule the LEB for scrubbing.

The volume table LEBs wont'd be read, though. To cover even that, you
one could introduce a special ioctl.

In case of fastmap, this ioctl would cover all the fatmap special LEBs
too.

>   But I would expect that because I'd have a hard
>   (impossible?) time doing an atomic read/re-write of a
>   LEB (or page), the UBIfs and my scrubber would interact
>   badly with my scrubber eventually corrupting the UBIfs
>   file system. Is there any easy way to interlock these
>   accesses (from the UBIfs and from my UBI-level scrubber)?
>   A way to temporarily suspend activity from the UBIfs?
> 
>   One kludge that might work is that I'm operating in a
>   real-time environment. If I made my scrubbing requests
>   from a very high priority (higher than the "System"
>   tasks that run around Priority 50), could I be sure
>   my read + rewrite scrubbing requests would at least
>   enter the UBI's work queue immediately adjacent to
>   each other (and without UBIfs requests intermingled)?
> 
>   Alternatively, I could probably dismount the UBIfs
>   before doing scrubbing, but I'd rather not have to
>   do that.

You do not need to do unmount anything with the above technique. If,
say, UBI scheduled an LEB 0:1 (volume 0, LEB 1) for scrubbing, and UBIFS
(which has volume 0 mounted) writes to LEB 1, UBI will block the writer
until the scrubbing finishes, so you do not have to worry.

HTH.

-- 
Best Regards,
Artem Bityutskiy