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    SubjectRe: Bug: Discontigmem virt_to_page() [Alpha,ARM,Mips64?]
    On Wed, May 01, 2002 at 04:41:17AM +0200, Daniel Phillips wrote:
    > On Thursday 02 May 2002 03:43, Andrea Arcangeli wrote:
    > > On Wed, May 01, 2002 at 03:26:22AM +0200, Daniel Phillips wrote:
    > > > For your information, the mem_map lives in *virtual* memory, it does not
    > > > need to change location, only the kernel page tables need to be updated,
    > > > to allow a section of kernel memory to be moved to a different physical
    > > > location. For user memory, this was always possible, now it is possible
    > > > for kernel memory as well. Naturally, it's not all you have to do to get
    > > > hotplug memory, but it's a big step in that direction.
    > >
    > > what kernel pagetables?
    > The normal page tables that are used to map kernel memory.
    > > pagetables for space that you left free for what?
    > These page tables have not been left free for anything. The nice thing about
    > page tables is that you can change the page table entries to point wherever
    > you want. (I know you know this.) This is what config_nonlinear supports,
    > and that is why it's called config_nonlinear. When we want to remap part of
    > the kernel memory to a different piece of physical memory, we just need to
    > fill in some pte's. The tricky part is knowing how to fill in the ptes, and
    > config_nonlinear takes care of that.
    > > You waste virtual space for that at the very least on x86 that is
    > > just very tigh, at this point kernel virtual space is more costly than
    > > physical space these days. And nevertheless most archs doesn't have
    > > pagetables at all to read and write the page structures. yes it's
    > > virtual memory but it's a direct mapping.
    > Most architectures? That's quite possibly an exaggeration. Some
    > architectures - MIPS32 for example - make this difficult or impossible,
    > and so what? Those can't do software hotplug memory, sorry.

    alpha is the same as mips I think. sparc would be the same too if
    there's any discontigmem sparc. Dunno of arm. We're talking about
    architectures needing discontigmem, 99% percent of users doesn't need
    discontigmem in the first place, you never need discontigmem in x86 and
    even in new-numa you don't need discontigmem, you want to pass through
    discontigmem only to get the numa topology description that the current
    discontigmem provides via the pgdat.

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