Re: [PATCH 3/4] mm/gup: Remove enfornced COW mechanism

Peter Xu
2020-09-14T18:00:00Z

On Mon, Sep 14, 2020 at 04:27:24PM +0200, Oleg Nesterov wrote:
> On 08/21, Peter Xu wrote:
> >
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -381,22 +381,13 @@ static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
> > }
> >
> > /*
> > - * FOLL_FORCE or a forced COW break can write even to unwritable pte's,
> > - * but only after we've gone through a COW cycle and they are dirty.
> > + * FOLL_FORCE can write to even unwritable pte's, but only
> > + * after we've gone through a COW cycle and they are dirty.
> > */
> > static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
> > {
> > - return pte_write(pte) || ((flags & FOLL_COW) && pte_dirty(pte));
> > -}
> > -
> > -/*
> > - * A (separate) COW fault might break the page the other way and
> > - * get_user_pages() would return the page from what is now the wrong
> > - * VM. So we need to force a COW break at GUP time even for reads.
> > - */
> > -static inline bool should_force_cow_break(struct vm_area_struct *vma, unsigned int flags)
> > -{
> > - return is_cow_mapping(vma->vm_flags) && (flags & (FOLL_GET | FOLL_PIN));
> > + return pte_write(pte) ||
> > + ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
>
> Do we really need to add the FOLL_FORCE check back?
>
> Afaics, FOLL_COW is only possible if FOLL_FORCE was set.

When I proposed the patch I wanted to add back FOLL_FORCE because the previous
removing of FOLL_FORCE should be related to the enforced COW mechanism where
FOLL_COW can definitely happen without FOLL_FORCE. So when we want to revert
the enforced COW we definitely need to recover this check too as it was. I
didn't think deeper than that.

However now I'm a bit confused on why FOLL_COW must be with FOLL_FORCE even
without the enforced COW... Shouldn't FOLL_COW be able to happen even without
FOLL_FORCE (as long as when a page is shared, and the gup is with WRITE
permission)? Not sure what I've missed, though.

--
Peter Xu