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

David Rientjes
2012-11-16T20:10:43Z

On Fri, 16 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.
>
> Now we have huge zero page fully implemented inside mm/huge_memory.c. Push
> this logic to pmd_modify() means we expose hzp implementation details to
> arch code. Looks ugly for me.
>

So you are suggesting that anybody who ever does pmd_modify() in the
future is responsible for knowing about the zero page and to protect
against giving it write permission in the calling code??