Re: How to map high memory for block io

Pierre Ossman
2006-01-27T20:29:04Z

Russell King wrote:
> On Fri, Jan 27, 2006 at 09:04:33PM +0100, Pierre Ossman wrote:
>
>> Russell King wrote:
>>
>>> I don't see what the problem is. A sg entry is a list of struct page
>>> pointers, an offset, and a size. As such, it can't describe a transfer
>>> which crosses a page because such a structure does not imply that one
>>> struct page follows another struct page.
>>>
>> If the pages do not strictly follow each other then there is a lot of
>> broken code in the kernel. drivers/mmc/mmci.c and drivers/block/ub.c
>> being two occurences since both assume they can access the entire entry
>> through a single mapping.
>>
>
> We don't make that assumption. What we do is:
>
> - map the current sg using kmap_atomic()
> - copy up to sg->length into or out of that mapping
> - unmap current sg
> - if we have reached the end of this sg, move on to the next
>

This it the algorithm I've been following, but...

> What this means is that we assume sg->offset + sg->length <= PAGE_SIZE
>

This doesn't seem to be true. Since when tracking this problem I added
the following:

BUG_ON((host->cur_sg->length + host->cur_sg->offset) > PAGE_SIZE);

which subsequently triggered. That was the start of my quest for how to
do highmem properly.

Rgds
Pierre