> On Wed, Mar 01 2006, Andy Chittenden wrote:
> > > On Wed, Mar 01 2006, Andi Kleen wrote:
> > > > On Wednesday 01 March 2006 15:34, Jens Axboe wrote:
> > > >
> > > >
> > > > > > It shouldn't end up with more, only with less.
> > > > >
> > > > > Sure yes, but if that 'less' is still more than what the
> > > driver can
> > > > > handle, then there's a problem.
> > > >
> > > > The driver needs to handle the full list it passed in.
> It's quite
> > > > possible that the iommu layer is unable to merge anything.
> > > >
> > > >
> > > > This isn't the block layer based merging where we guarantee
> > > > to be able to merge in advance - just lazy after the
> fact merging.
> > >
> > > Yes I realize that, I wonder if the bounce patch screwed
> something up
> > > that destroys the block layer merging/accounting. We'll
> know when Andy
> > > posts results that dump the request as well.
> >
> > And here's the dmesg o/p (I had to gather it from
> /var/log/kern.log as
> > there was so much output):
>
> Thanks!
>
> > hda: DMA table too small
> > ide dma table, 256 entries, bounce pfn 1310720
> > sg0: dma=830e800, len=4096/0, pfn=1202633
> > sg1: dma=830f800, len=4096/0, pfn=1202590
> > sg2: dma=8310800, len=4096/0, pfn=1202548
> > sg3: dma=8311800, len=4096/0, pfn=1202506
>
> Alright Andi, take a look at this then. We have the same thing again,
> mid page start of the sg entries. The block layer has done no merging,
> it's 256 separate segments. The pci_map_sg() output is the
> same, except
> that the IDE driver now needs to split the entries. The
> corresponding rq
> entries for the first four above are:
>
> > request: phys seg 256, hw seg 256, nr_sectors 2048
> > bio0: bytes=4096, phys seg 1, hw seg 1
> > bvec0: addr=ffff8101259c9000, size=4096, off=0
> > bio1: bytes=4096, phys seg 1, hw seg 1
> > bvec0: addr=ffff81012599e000, size=4096, off=0
> > bio2: bytes=4096, phys seg 1, hw seg 1
> > bvec0: addr=ffff810125974000, size=4096, off=0
> > bio3: bytes=4096, phys seg 1, hw seg 1
> > bvec0: addr=ffff81012594a000, size=4096, off=0
>
> these here. Totally plain 4kb bios strung to the request, no funky
> offsets or anything. 256 hardware and physical segments, for
> a total of
> a 1MB request.
>
> So what is going wrong? Why does the pci mapping output looks so
> "strange"?
>
> --
> Jens Axboe
>
So, what's the story? Apart from the performance implications, one thing
that does concern me is whether this could cause corruptions on disk as
it appears a cross-page write is being attempted.
--
Andy, BlueArc Engineering
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