Re: [PATCH 1/1] mm/hugetlb: Make huge_pte_offset() consistent and document behaviour

[Michal Hocko]

On Wed 26-07-17 10:50:38, Michal Hocko wrote:
> On Tue 25-07-17 16:41:14, Punit Agrawal wrote:
> > When walking the page tables to resolve an address that points to
> > !p*d_present() entry, huge_pte_offset() returns inconsistent values
> > depending on the level of page table (PUD or PMD).
> > 
> > It returns NULL in the case of a PUD entry while in the case of a PMD
> > entry, it returns a pointer to the page table entry.
> > 
> > A similar inconsitency exists when handling swap entries - returns NULL
> > for a PUD entry while a pointer to the pte_t is retured for the PMD
> > entry.
> > 
> > Update huge_pte_offset() to make the behaviour consistent - return NULL
> > in the case of p*d_none() and a pointer to the pte_t for hugepage or
> > swap entries.
> > 
> > Document the behaviour to clarify the expected behaviour of this
> > function. This is to set clear semantics for architecture specific
> > implementations of huge_pte_offset().
> 
> hugetlb pte semantic is a disaster and I agree it could see some
> cleanup/clarifications but I am quite nervous to see a patchi like this.
> How do we check that nothing will get silently broken by this change?

Forgot to add. Hugetlb have been special because of the pte sharing. I
haven't looked into that code for quite some time but there might be a
good reason why pud behave differently.
-- 
Michal Hocko
SUSE Labs