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

Kirill A. Shutemov
2012-11-16T18:12:15Z

[-- Attachment #1: Type: text/plain, Size: 1700 bytes --]

On Thu, Nov 15, 2012 at 01:47:33PM -0800, David Rientjes wrote:
> 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.

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.

> > 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.

I haven't found where we check if the page is a 4k zero page, but it's
definitely not pte_modify().

--
Kirill A. Shutemov

[-- Attachment #2: Digital signature --]
[-- Type: application/pgp-signature, Size: 836 bytes --]