Re: How to map high memory for block io

Pierre Ossman
2006-01-30T07:57:58Z

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.

(Readding Russell as cc since he seems to have gotten lost somewhere)