Re: [PATCH v5 05/11] thp: change_huge_pmd(): keep huge zero page write-protected

David Rientjes
2012-11-15T21:47:36Z

On Thu, 15 Nov 2012, Kirill A. Shutemov wrote:

> > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > > index d767a7c..05490b3 100644
> > > --- a/mm/huge_memory.c
> > > +++ b/mm/huge_memory.c
> > > @@ -1259,6 +1259,8 @@ int change_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd,
> > > pmd_t entry;
> > > entry = pmdp_get_and_clear(mm, addr, pmd);
> > > entry = pmd_modify(entry, newprot);
> > > + if (is_huge_zero_pmd(entry))
> > > + entry = pmd_wrprotect(entry);
> > > set_pmd_at(mm, addr, pmd, entry);
> > > spin_unlock(&vma->vm_mm->page_table_lock);
> > > ret = 1;
> >
> > Nack, this should be handled in pmd_modify().
>
> I disagree. It means we will have to enable hzp per arch. Bad idea.
>

pmd_modify() only exists for those architectures with thp support already,
so you've already implicitly enabled for all the necessary architectures
with your patchset.

> What's wrong with the check?
>

Anybody using pmd_modify() to set new protections in the future perhaps
without knowledge of huge zero page can incorrectly make the huge zero
page writable, which can never be allowed to happen. It's better to make
sure it can never happen with the usual interface to modify protections.