Re: [BUG] hugetlb MAP_PRIVATE mapping vs /dev/zero

[Andrew Morton]

Oleg Nesterov <oleg@xxxxxxxxxx> wrote:
>
> Hugetlbfs mmap with MAP_PRIVATE becomes MAP_SHARED
> silently, but vma->vm_flags have no VM_SHARED bit.
> I think it make sense to forbid MAP_PRIVATE in
> hugetlbfs_file_mmap() because it may confuse user
> space applications. But the real bug is that reading
> from /dev/zero into hugetlb will do:
> 
> read_zero()
> 	read_zero_pagealigned()
> 		if (vma->vm_flags & VM_SHARED)
> 			break;	// OK if MAP_PRIVATE
> 		zap_page_range();
> 		zeromap_page_range();
> 
> We can fix hugetlbfs_file_mmap() or read_zero_pagealigned()
> or both.

This could break existing applications, yes?

If so, would an appropriate fixup be to coerce private mappings
of hugetlb files into shared ones in-kernel?
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