Re: [patch 05/16] mm: Allow special mappings with user access cleared

[Peter Zijlstra]

On Wed, Dec 13, 2017 at 03:57:40PM +0300, Kirill A. Shutemov wrote:
> On Wed, Dec 13, 2017 at 01:22:11PM +0100, Peter Zijlstra wrote:

> > get_user_pages_fast() will ultimately end up doing
> > pte_access_permitted() before getting the page, follow_page OTOH does
> > not do this, which makes for a curious difference between the two.
> > 
> > So I'm thinking we want the below irrespective of the VM_NOUSER patch,
> > but with VM_NOUSER it would mean write(2) will no longer be able to
> > access the page.
> 
> Oh..
> 
> We do call pte_access_permitted(), but only for write access.
> See can_follow_write_pte().

My can_follow_write_pte() looks like:

static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
{
	return pte_write(pte) ||
		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
}

am I perchance looking at the wrong tee?

> The issue seems bigger: we also need such calls for other page table levels :-/

Sure.