lkml.org 
[lkml]   [2012]   [Nov]   [20]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v5 05/11] thp: change_huge_pmd(): keep huge zero page write-protected
On Fri, Nov 16, 2012 at 12:10:39PM -0800, David Rientjes wrote:
> 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??

Looks like we don't need the patch at all.

IIUC, if you ask for PROT_WRITE vm_get_page_prot() will translate it to
_PAGE_COPY or similar and you'll only get the page writable on pagefault.

Could anybody confirm that it's correct?

--
Kirill A. Shutemov
[unhandled content-type:application/pgp-signature]
\
 
 \ /
  Last update: 2012-11-20 17:41    [W:0.095 / U:0.184 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site