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    SubjectRe: How to map high memory for block io
    On Fri, Jan 27 2006, 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.

    Assuming that page idx and idx + 1 in a bio are contigious is a very bad
    one, no one has ever made any such guarantees. Besides, if you kmap()
    it, things change. The pages in a bio have nothing to do with each
    other, they are usually just a consequence of how the page cache ended
    up allocating. So completely random usually, once you get a little

    Are you sure you are reading that code correctly? Code making such
    assumptions would be breaking pretty quickly.

    Jens Axboe

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      Last update: 2006-01-27 21:11    [W:0.021 / U:5.752 seconds]
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