linux-kernel - Re: [PATCH 4.4 15/47] ubi: fastmap: Correctly handle interrupted erasures in EBA

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Date:   Thu, 26 Jul 2018 08:25:36 +0200
From:   Richard Weinberger <richard@....at>
To:     Ben Hutchings <ben.hutchings@...ethink.co.uk>
Cc:     martin bayern <Martinbayern@...look.com>, stable@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4.4 15/47] ubi: fastmap: Correctly handle interrupted erasures in EBA

Ben,

Am Donnerstag, 26. Juli 2018, 04:12:54 CEST schrieb Ben Hutchings:
> On Tue, 2018-07-10 at 20:24 +0200, Greg Kroah-Hartman wrote:
> > 4.4-stable review patch.  If anyone has any objections, please let me 
know.
> > 
> > ------------------
> > 
> > From: Richard Weinberger <richard@....at>
> > 
> > commit 781932375ffc6411713ee0926ccae8596ed0261c upstream.
> > 
> > Fastmap cannot track the LEB unmap operation, therefore it can
> > happen that after an interrupted erasure the mapping still looks
> > good from Fastmap's point of view, while reading from the PEB will
> > cause an ECC error and confuses the upper layer.
> > 
> > Instead of teaching users of UBI how to deal with that, we read back
> > the VID header and check for errors. If the PEB is empty or shows ECC
> > errors we fixup the mapping and schedule the PEB for erasure.
> [...]
> 
> Isn't there a risk here, that a read error leads to erasing data that
> might be recoverable if the read is retried?

Well, read error means that already something went very wrong. At other places
in UBI wo also don't retry reading headers and consider it as fatal when we 
are unable to read it.
We could also read the EC header, but what do we gain from that?
If the VID header is not readable we cannot check fastmap either.

What case exactly do you have in mind?

Thanks,
//richard