Re: Bug: Discontigmem virt_to_page() [Alpha,ARM,Mips64?]

[Andrea Arcangeli <andrea@suse.de>]

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.

Andrea