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

[David Hildenbrand]

On 14.06.23 17:31, Peter Xu wrote:
> On Wed, Jun 14, 2023 at 05:17:13PM +0200, David Hildenbrand wrote:
> > 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
>
> I think it is still a "permission check"?  It means, we forbid anyone R/O
> taking the page if it's not exclusively owned, just like we forbid anyone
> RW taking the page if it's not writable?

Agreed, just not in the traditional PTE-protection case.

> It's just that the permission check only applies to PIN which follow_page()
> doesn't yet care, so it won't ever trigger.
>
> > 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".
>
> Sure.
>
> > As unsharing only applies to FOLL_PIN, it doesn't quite fit into patch #2.
> > Either move that to this patch or squash both.
>
> Sure, no strong opinions here.
>
> The plan is _if_ someone wants to backport patch 2, this patch should not
> be part of it.  But then maybe it makes more sense to move the CoR change
> there into this one, not because "it's not permission check", but because
> CoR is not relevant in follow_page(), so not relevant to a backport.

Right. Then just call patch #2 "Add missing write-permission check" and 
this patch "Support FOLL_PIN in hugetlb_follow_page_mask()" or sth. like 
that.

Regarding the backport, I really wonder if patch #2 is required at all, 
because I didn't sport any applicable FOLL_WRITE users. Maybe there were 
some? Hm. If it's not applicable, a single "Support FOLL_PIN in 
hugetlb_follow_page_mask()" patch might be cleanest.

--
Cheers,

David / dhildenb