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    SubjectRe: Creating a per-task kernel space for kmap, user pagetables, et al
    On Wed, 20 Mar 2002, Christoph Hellwig wrote:
    > On Wed, Mar 20, 2002 at 09:23:41PM +0100, Andrea Arcangeli wrote:
    > > we need to walk pagetables not just from the current task and mapping
    > > pagetables there would decrase the user address space too much.
    >
    > Who sais it should be taken from user address space?
    > For example openunix takes a small (I think 4MB) part of the normal KVA
    > to be per-process mapped.

    Linux would want it to come from the user address space because it has
    no precedent for per-process addressing in the kernel address space,
    and much simpler to keep it that way.

    > > I think you're missing the problem with mainline. There is no shortage
    > > of virtual address space, there is a shortage of physical ram in the
    > > zone normal. So we cannot keep them in zone normal (and there's no such
    > > thing as "mapping in zone_normal"). Maybe I misunderstood what you were
    > > saying.
    >
    > The problem is not the 4GB ZONE_NORMAL but the ~1GB KVA space.
    > UnixWare/OpenUnix had huge problems getting all kernel structs for managing
    > 16GB virtual into that - on the other hand their struct page is more
    > then twice as big as ours..

    Which is more good reason to put user-address-space-specific-mappings
    (page table mappings; the filepage mapping case is less obvious) in
    the user address space (but of course not accessible to the user) -
    probably above user stack, since that's already of indefinite size.

    Hugh

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