Re: [PATCH 2/7] mm/hugetlb: Fix hugetlb_follow_page_mask() on permission checks

[Mike Kravetz <mike.kravetz@oracle.com>]

On 06/14/23 11:46, Peter Xu wrote:
> On Wed, Jun 14, 2023 at 05:31:36PM +0200, David Hildenbrand wrote:
> > On 13.06.23 23:53, Peter Xu wrote:
> 
> Then I assume no fixes /backport needed at all (which is what this patch
> already does).  It's purely to be prepared only.  I'll mention that in the
> new version.

Code looks fine to me.  Feel free to add,

Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
-- 
Mike Kravetz

> > > 
> > > When at it, switching the try_grab_page() to use WARN_ON_ONCE(), to be
> > > clear that it just should never fail.
> > > 
> > > Signed-off-by: Peter Xu <peterx@redhat.com>
> > > ---
> > >   mm/hugetlb.c | 22 ++++++++++++++++------
> > >   1 file changed, 16 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > > index 82dfdd96db4c..9c261921b2cf 100644
> > > --- a/mm/hugetlb.c
> > > +++ b/mm/hugetlb.c
> > > @@ -6481,8 +6481,21 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
> > >   	ptl = huge_pte_lock(h, mm, pte);
> > >   	entry = huge_ptep_get(pte);
> > >   	if (pte_present(entry)) {
> > > -		page = pte_page(entry) +
> > > -				((address & ~huge_page_mask(h)) >> PAGE_SHIFT);
> > > +		page = pte_page(entry);
> > > +
> > > +		if (gup_must_unshare(vma, flags, page)) {
> > > +			/* Tell the caller to do Copy-On-Read */
> > > +			page = ERR_PTR(-EMLINK);
> > > +			goto out;
> > > +		}
> > > +
> > > +		if ((flags & FOLL_WRITE) && !pte_write(entry)) {
> > > +			page = NULL;
> > > +			goto out;
> > > +		}
> > > +
> > > +		page += ((address & ~huge_page_mask(h)) >> PAGE_SHIFT);
> > > +
> > >   		/*
> > >   		 * Note that page may be a sub-page, and with vmemmap
> > >   		 * optimizations the page struct may be read only.
> > > @@ -6492,10 +6505,7 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
> > >   		 * try_grab_page() should always be able to get the page here,
> > >   		 * because we hold the ptl lock and have verified pte_present().
> > >   		 */
> > > -		if (try_grab_page(page, flags)) {
> > > -			page = NULL;
> > > -			goto out;
> > > -		}
> > > +		WARN_ON_ONCE(try_grab_page(page, flags));
> > >   	}
> > >   out:
> > >   	spin_unlock(ptl);