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[Xen-devel] A better fix for the blkdev merging bug

> > I've given this change a go on my PE1650 (aacraid driver). 
> > Unfortunately this seems to be one of the SCSI drivers that doesn't 
> > correctly handle the error condition.
> > 
> > Running my usual test ('compare' in dom0, compiles in other domains), I 
> > don't see any differences in the compares, but after a few minutes, I 
> > get the following on the console, and everything is stuck waiting for 
> > disk.
> > 
> > aacraid: cmd len 00000000 cmd underflow 00010000
> > aacraid: Host adapter reset request. SCSI hang ?
> > 
> > The latter message repeats every few seconds. I rebooted the box with 
> > the Xen console after a few lines. I'm going to try the aacraid driver 
> > from 2.4.27-rc3, which I believe has had some attention recently.
> Looks like the SCSI-merge code will have to be modified. I'll do
> IDE-merge code at the same time, so that we just merge less
> aggressively rather than falling back to PIO transfers.
> I'll leave the check in pci_map_sg(), but it shouldn't ever trigger
> after I patch the IDE and SCSI merge routines, so I'll add a warning
> message if an invalid scatter-gather list is detected.

Okay, I've checked in a better fix for the blkdev merging bug, which
should work okay for all IDE and SCSI devices. Other advantages are
that it still permits merging wherever possible, and there is no need
for fallback to PIO transfers.

Please can people give this a try, particularly on SCSI - fingers
crossed the bug will be gone.

 -- Keir

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