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    SubjectRe: How to map high memory for block io
    On Fri, Jan 27 2006, 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
    > What this means is that we assume sg->offset + sg->length <= PAGE_SIZE
    > in all cases, which is the same assumption architecture DMA code makes.
    > If that's invalid, there's likely to be a lot of architecture DMA support
    > which is broken.

    That is definitely valid, same goes for the bio_vec structure. They map
    _a_ page, after all :-)

    Jens Axboe

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