Re: mprotect and hugetlb mappings

[Matthew Wilcox]

On Wed, Jul 05, 2023 at 04:08:08PM -0700, Mike Kravetz wrote:
> I was recently asked about the behavior of mprotect on a hugetlb
> mapping where addr or addr+len is not hugetlb page size aligned.  As
> one might expect, EINVAL is returned in such cases.  However, the man
> page makes no mention of alignment requirements for hugetlb mappings.
> 
> I am happy to submit man page updates if people agree this is the correct
> behavior.  We might even want to check alignment earlier in the code
> path as we fail when trying to split the vma today.
> 
> An alternative behavior would be to operate on whole hugetlb pages within
> the range addr - addr+len.

After a careful re-reading of the mprotect() man page, I suggest the
following behaviour ...

addr must be a multiple of the hpage size.  Otherwise -EINVAL.
len should be rounded up to hpage size.

I wonder how likely this change would be to break userspace code.
Maybe some test cases.