Re: Why preallocate pmd in x86 32-bit PAE?

[Jeremy Fitzhardinge <jeremy@goop.org>]

Linus Torvalds wrote:
> On Thu, 15 Nov 2007, Jeremy Fitzhardinge wrote:
>   
>> Once difference is that 64-bit incrementally allocates all levels of the
>> pagetable, whereas 32-bit PAE preallocates the 4 pmds when it allocates
>> the pgd.  What's the rationale for this?  What pitfalls would there be
>> in making them incrementally allocated?
>>     
>
> IIRC, the present bit is ignored in the magic 4-entry PGD.  All entries 
> have to be present.
>   

Hm, do you recall what processors that might affect?  As far as I know,
current processors will ignore non-present top-level entries.  Anyway,
we can point them not present to empty_zero_page, so testing the present
bit will still be sufficient to tell if we need to allocate a new pmd,
but if the hardware decides to follow the page reference there's no harm
done.  (Hm, unless the hardware decides it wants to set A or D bits in
empty_zero_page for some reason...)

> What earlier CPU's did was to basically load all four values into the CPU 
> when you loaded %cr3. There was no "three-level page table walker" at all: 
> it was still a two-level page table walker, there were just for magic 
> internal page tables that were indexed off the two high bits.
>   

That just means we need to reload cr3 after populating the pgd with a
new pmd, right?

    J