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