>
>On Wed, 28 Oct 1998, Linus Torvalds wrote:
>
>> layer how complicates it oevrmuch for little gain. The generated code was
>> fairly horrible when everything checked the 4M bit in the PMD (it's
>> actualy in the pgd, but I used the pmd macros to clean up the conceptual
>> idea).
>
>i think the check could be done within pmd_bad() [pgd_bad()] conditions,
>which branches already exist for all relevant cases. Thus the code path
>would not differ for normal mappings.
This was my thought too. pmd_only() would be something like:
#define pmd_bad(x) ((pmd_val(x) & (~PAGE_MASK & ~_PAGE_USER)) !=
(_KERNPG_TABLE | _PAGE_4M))
So we can do:
if (pmd_bad(pmd))
{
/*
* We usually don' t get here.
*/
if (pmd_only(pmd))
copy it and break;
else {
clear_pte()
...
}
}
(i removed the global flag here but this is only an example. This way it'
s possible to do every check in the pmd_bad() path). I am _not_ going to
implement this following Linus's hint of course ;-).
Another solution could be to implement a very arch specific i386 syscall
that give us a 4 mbyte view (not allocation) of some piece of phisical
memory. Maybe is a bad idea ;-).
Andrea Arcangeli
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