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    SubjectRe: swsusp with highmem, testing wanted
    On Thu, Mar 25 2004, Pavel Machek wrote:
    > Hi!
    >
    > > > Having special "poll" mode for block drivers might do the trick, but
    > > > thats lot of work.
    > >
    > > Maybe I'm missing something, but why doesn't the regular io paths
    > > work?
    >
    > (Basically) you want to replace all kernel data with kernel data saved
    > on disk. How do you do that using normal i/o paths? If you'll read
    > "new" data 4KB at a time, you'll crash... because you still need "old"
    > data to do the reading, and "new" data may fit on same physical spot
    > in memory.
    >
    > There are two solutions to this:
    >
    > * require half of memory to be free during suspend. That way you can
    > read "new" data onto free spots, then cli and copy
    >
    > * assume we had special "polling" ide driver that only uses memory
    > between 0-640KB. That way, I'd have to make sure that 0-640KB is free
    > during suspending, but otherwise it would work...
    >
    > Do you see the problem now?

    I see what you mean.

    > > > Which operations are allowed to access highmem? Can I rely on
    > > > block device read/write not accessing highmem?
    > >
    > > You mean modify highmem pages, or?
    >
    > I'd like to know this. Suppose I ask block subsystem to read from disk
    > into page @1.8GB. All the highmem contains trash. Will block subsystem
    > be able to work in this situation?

    We've never enforced anything like that, so you cannot rely on it. Block
    layer itself doesn't keep anything in high memory, and I cannot imagine
    any drivers that do either.

    --
    Jens Axboe

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