Re: [patch 2/6] CONFIG_HIGHPTE vs. sub-page page tables.

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Fri, 2007-10-26 at 09:42 +0200, Martin Schwidefsky wrote:
> On Fri, 2007-10-26 at 06:47 +1000, Benjamin Herrenschmidt wrote:
> > > Solution: The only solution I found to this dilemma is a new typedef:
> > > a pgtable_t. For s390 pgtable_t will be a (pte *) - to be introduced
> > > with a later patch. For everybody else it will be a (struct page *).
> > > The additional problem with the initialization of the ptl lock and the
> > > NR_PAGETABLE accounting is solved with a constructor pgtable_page_ctor
> > > and a destructor pgtable_page_dtor. The page table allocation and free
> > > functions need to call these two whenever a page table page is allocated
> > > or freed. pmd_populate will get a pgtable_t instead of a struct page
> > > pointer. To get the pgtable_t back from a pmd entry that has been
> > > installed with pmd_populate a new function pmd_pgtable is added. It
> > > replaces the pmd_page call in free_pte_range and apply_to_pte_range.
> >
> > Interesting. That means I don't need to have a PTE page to be a struct
> > page anymore ? I can have good use for that on powerpc as well... 
> 
> That would be good news. I'm curious, can you elaborate on what the use
> case is?

When using 64K pages, we use 32K of PTEs and 32K of "extension". The
extension thing is used when using HW 4K pages, to keep track of the
subpages. On setups where that isn't needed, we can save memory by
allocating half pages...

Ben.

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