Re: How to map high memory for block io

[Jens Axboe <axboe@suse.de>]

On Mon, Jan 30 2006, Pierre Ossman wrote:
> Jens Axboe wrote:
> > On Sat, Jan 28 2006, Pierre Ossman wrote:
> >   
> >> Jens Axboe wrote:
> >>     
> >>> On Fri, Jan 27 2006, Russell King wrote:
> >>>   
> >>>       
> >>>> On Fri, Jan 27, 2006 at 10:58:59PM +0100, Pierre Ossman wrote:
> >>>>     
> >>>>         
> >>>>> Test done here, few minutes ago. Added this to the wbsd driver in its
> >>>>> kmap routine:
> >>>>>
> >>>>>     if ((host->cur_sg->offset + host->cur_sg->length) > PAGE_SIZE)
> >>>>>         printk(KERN_DEBUG "wbsd: Big sg: %d, %d\n",
> >>>>>             host->cur_sg->offset, host->cur_sg->length);
> >>>>>
> >>>>> got:
> >>>>>
> >>>>> [17385.425389] wbsd: Big sg: 0, 8192
> >>>>> [17385.436849] wbsd: Big sg: 0, 7168
> >>>>> [17385.436859] wbsd: Big sg: 0, 7168
> >>>>> [17385.454029] wbsd: Big sg: 2560, 5632
> >>>>> [17385.454216] wbsd: Big sg: 2560, 5632
> >>>>>       
> >>>>>           
> >>>> Jens - what's going on?  These look like invalid sg entries to me.
> >>>>
> >>>> If they are supposed to be like that, there will be additional problems
> >>>> for block drivers ensuring cache coherency on PIO.
> >>>>     
> >>>>         
> >>> No freaking idea, must be coming out of the pci dma mapping. The IOMMU
> >>> doing funky stuff? How are these sg lists mapped?
> >>>
> >>>   
> >>>       
> >> This is an ISA (i.e. platform) device, so no PCI involved. There is also
> >> no IOMMU on this system.
> >>
> >> As for the mapping there doesn't seem to be anything fancy about it
> >> (this is Russell's territory so this is just my naive view of it). The
> >> queue is set up in mmc_queue.c and the sg is mapped using
> >> blk_rq_map_sg() in mmc_block.c.
> >>
> >> But if sg entries are not supposed to cross pages, then I guess that
> >> means that any transfer is limited in size by PAGE_SIZE *
> >> min(max_phys_seg, max_hw_seg), right?
> >>     
> >
> > Ah, you need to disable clustering to prevent that from happening! I was
> > confused there for a while.
> >
> >   
> 
> And which is the lesser evil, highmem bounce buffers or disabling
> clustering? I'd probably vote for the former since the MMC overhead can
> be quite large.

Disabling clustering is by far the least expensive way to accomplish it.

-- 
Jens Axboe

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