Re: [PATCH] drivers/scsi/sd.c: fix uninitialized variable in handling medium errors

[Andrew Morton <akpm@osdl.org>]

Mark Lord <lkml@rtr.ca> wrote:
>
> > That's if we think -stable needs this fixed.
> 
> Let's say a bunch of read bio's get coalesced into a single
> 200+ sector request.  This then fails on one single bad sector
> out of the 200+.  Without the patch, there is a very good chance
> that sd.c will simply fail the entire request, all 200+ sectors.
> 
> With the patch, it will fail the first block, and then retry
> the remaining blocks.  And repeat this until something works,
> or until everything has failed one by one.

Yowch.  I have the feeling that this'll take our EIO-handling time from
far-too-long to far-too-long*200.

I am still traumatised by my recent ten-minute wait for a dodgy DVD to
become ejectable.

I don't think -stable needs this, personally.

> Better, but still not the best.
> 
> What I need to have happen when a request is failed due to bad-media,
> is have it split the request into a sequence of single-block requests
> that are passed to the LLD one at a time.  The ones with real bad
> sectors will then be independently failed, and the rest will get done.
> 
> Much better.  Much more complex.
> 
> I'm thinking about something like that, just not sure whether to put it
> (initially) in libata, sd.c, or the block layer.

block, I suspect.  My DVD trauma was IDE-induced.  Jens is mulling the
problem - I'd suggest you coordinate with him.

It would be a good thing to fix.

It's moderately hard to test, though.  Easy enough for DVDs and CDs, but
it's harder to take a marker pen to a hard drive.