[parisc-linux] Proposal for altering our Page Table layouts

[James Bottomley]

On Mon, 2004-04-12 at 18:31, John Marvin wrote:
> Let's not forget that the machine independent VM code assumes a 2 or 3
> level page table, walks those page tables, allocates page tables, etc.
> So, let's forget about theory for a minute, and start talking
> realistically.  Have you considered how you would abstract an inverted
> page table design so that it would fit within the machine independent VM
> design for page table support?  I haven't given it more than about 5
> minutes of thought, but I don't see a way of doing it (Note that I am
> not for this idea at all).

I think we can all agree that the results presented in the paper show
(albeit indirectly) that IPT performs worse than a 2 level page table
(FMPT in the paper).

However, as far as linux goes, the abstraction would actually cover a 1
level page table as well ... and we could make a HPT directly emulate a
1-level table as long as we did the hash chain walking within the
pgd_offset macro.

> If you can't do it (i.e. hide it completely within the parisc arch code) then
> you need to be talking to Linus and convince him first, unless you are
> advocating maintaining a large patch against machine independent code.

However, realistically, I think a global HPT is incompatible with the
way linux does VM and a local HPT (one per process) while possible, gets
us into awful memory allocation problems to the extent that it's not
worth bothering with.  Therefore, I think a 2 level table will be
optimal for us.

James