Re: [PATCH 4/7] mm/hugetlb: Prepare hugetlb_follow_page_mask() for FOLL_PIN

[David Hildenbrand]

On 14.06.23 17:11, Peter Xu wrote:
> On Wed, Jun 14, 2023 at 04:57:37PM +0200, David Hildenbrand wrote:
> > On 13.06.23 23:53, Peter Xu wrote:
> > > It's coming, not yet, but soon.  Loose the restriction.
> > >
> > > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> > > ---
> > >    mm/hugetlb.c | 7 -------
> > >    1 file changed, 7 deletions(-)
> > >
> > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> > > index f037eaf9d819..31d8f18bc2e4 100644
> > > --- a/mm/hugetlb.c
> > > +++ b/mm/hugetlb.c
> > > @@ -6467,13 +6467,6 @@ struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
> > >    	spinlock_t *ptl;
> > >    	pte_t *pte, entry;
> > > -	/*
> > > -	 * FOLL_PIN is not supported for follow_page(). Ordinary GUP goes via
> > > -	 * follow_hugetlb_page().
> > > -	 */
> > > -	if (WARN_ON_ONCE(flags & FOLL_PIN))
> > > -		return NULL;
> > > -
> > >    	hugetlb_vma_lock_read(vma);
> > >    	pte = hugetlb_walk(vma, haddr, huge_page_size(h));
> > >    	if (!pte)
> > Did you fix why the warning was placed there in the first place? (IIRC, at
> > least unsharing support needs to be added, maybe more)
>
> Feel free to have a look at patch 2 - it should be done there, hopefully in
> the right way.  And IIUC it could be a bug to not do that before (besides
> CoR there was also the pgtable permission checks that was missing).  More
> details in patch 2's commit message.  Thanks,

Oh, that slipped my eyes (unsharing is not really a permission check) -- 
and the patch description could have been more explicit about why we can 
now lift the restrictions.

For the records: we don't use CoR terminology upstream. As suggested by 
John, we use "GUP-triggered unsharing".

As unsharing only applies to FOLL_PIN, it doesn't quite fit into patch 
#2. Either move that to this patch or squash both.

--
Cheers,

David / dhildenb