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

[Jeremy Fitzhardinge]

Linus Torvalds wrote:
> On Fri, 16 Nov 2007, Jeremy Fitzhardinge wrote:
>   
>> If you happened to do a 1G munmap, it may end up freeing and 
>> reallocating them, but that's going to be very rare.
>>     
>
> I don't think we ever free the pmd's now, do we?
>
> (Except for the *final* free, of course, when we release the whole VM).

Not for 32-bit at the moment, but it does in principle.  munmap ends up
calling free_pgtables, and so ends up calling pmd_free_range.  That will
do a pud_clear to detach the pmd from the pagetable and call
__pmd_free_tlb, which ends up doing tlb_remove_page ->
free_page_and_swap_cache.  32-bit knobbles all this at the moment, but
it looks to me like it wouldn't be hard to make this work if the code is
all common with 64-bit.

    J
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