linux-kernel - Re: UBI fastmap updates

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Message-ID: <20120802183210.7076aa48@spider.haslach.nod.at>
Date:	Thu, 2 Aug 2012 18:32:10 +0200
From:	Richard Weinberger <richard@....at>
To:	artem.bityutskiy@...ux.intel.com
Cc:	linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org,
	adrian.hunter@...el.com, Heinz.Egger@...utronix.de,
	thomas.wucher@...utronix.de, shmulik.ladkani@...il.com,
	tglx@...utronix.de, tim.bird@...sony.com, Marius.Mazarel@...l.ro,
	nyoushchenko@...sta.com
Subject: Re: UBI fastmap updates

Am Thu, 02 Aug 2012 19:17:47 +0300
schrieb Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>:

> On Thu, 2012-08-02 at 16:51 +0200, Richard Weinberger wrote:
> > Every time fastmap writes a new fastmap to the flash it tries to
> > get a new PEB and returns the old one (used for the old fastmap)
> > back to the WL sub-system.
> 
> OK.
> 
> > If no free PEBs are available (E.g Volume is full or the erase
> > worker is too slow) ubi_wl_get_fm_peb() returns NULL and fastmap
> > reuses the currently used PEB.
> 
> This should not happen. Fastmap should _reserve_ enough of PEBs for it
> to operate. It should always find the PEB to write.

What is the benefit?
IOW what is wrong with the current approach?

> Just like if you create a volume maximum possible size, we guarantee
> that you can fill it with data, and UBI will find enough PEBs for
> that.
> 
> Just like we always have enough PEBs for the volume table.
> 
> The above things are UBI's liabilities.
> 
> In the situation when a lot of PEBs became bad, UBI will switch to R/O
> mode with a scary message if it notices that it does not have enough
> PEBs to satisfy all the liabilities.
> 
> And this is why we reserve 2% of PEBs for bad PEBs handling.

Of course fastmap could also do something like that, but I don't really
see a benefit in this.

> > In this situation ubi_wl_get_fm_peb() may trigger such an error
> > message. If think we should get rid of the message as this is not
> > an error condition. It's a well known execution path.
> 
> Unless I am confused, this should lead to switching to R/O mode
> instead, just like we do when we write to an LEB and do not find a
> PEB to map to.

Why?
If everything goes wrong, fastmap makes sure that no fastmap is on
flash.
In case of a powercut we fall back to scanning mode.
R/O mode is overkill IMHO.

Thanks,
//richard

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