On Wed, Mar 01 2006, Andi Kleen wrote:
> On Wednesday 01 March 2006 14:41, Jens Axboe wrote:
> > On Wed, Mar 01 2006, Andy Chittenden wrote:
> > > with revised patch that does:
> > >
> > > printk("sg%d: dma=%llx, dma_len=%u/%u, pfn=%lu\n", i,
> > > (unsigned long long) sg->dma_address, sg->dma_length, sg->offset,
> > > page_to_pfn(sg->page));
> >
> > That is correct, thanks!
> >
> > > hda: DMA table too small
> > > ide dma table, 255 entries, bounce pfn 1310720
> > > sg0: dma=81c8800, dma_len=4096/0, pfn=1296369
> >
> > Still the same badness here, it's 2kb into a page so straddles two pages
> > for one entry.
>
> That's normal if it was in the IOMMU and a merged entry.
>
> You can try iommu=nomerge.
>
> Or maybe the higher layers are passing in physically continuous pages
> that get merged? Not too unlikely at boot.
But that would have to be 1kb or 512b io going in, I would expect 4kb to
be the base entries for any normal type of setup (with a 4kb fs). The
8kb could be one such merged entry, I agree.
> > Andi, any idea what is going on here? Why is this throwing up all of a
> > sudden??
>
> What is throwing up exactly?
>
> There was a change recently in the merging algorithm, but it shouldn't
> cause any bad side effects for correct users of *_map_sg()
That the request we end up passing to blk_rq_map_sg() and then to
pci_map_sg() ends up with more entries than the driver advertised. So
far I think only Andy reported this, and then only with your
blk_queue_bounce_limit() patch applied.
--
Jens Axboe
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