Re: [PATCH] mm: Respect FOLL_FORCE/FOLL_COW for thp

[Kirill A. Shutemov]

On Tue, Jan 10, 2017 at 11:06:10PM -0800, Hugh Dickins wrote:
> On Tue, 10 Jan 2017, Hugh Dickins wrote:
> > On Thu, 5 Jan 2017, Kirill A. Shutemov wrote:
> > > On Thu, Jan 05, 2017 at 12:36:58AM -0500, Keno Fischer wrote:
> > > >  struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
> > > >  		pmd_t *pmd, int flags)
> > > >  {
> > > > @@ -783,7 +793,7 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
> > > >  
> > > >  	assert_spin_locked(pmd_lockptr(mm, pmd));
> > > >  
> > > > -	if (flags & FOLL_WRITE && !pmd_write(*pmd))
> > > > +	if (flags & FOLL_WRITE && !can_follow_write_pmd(*pmd, flags))
> > > >  		return NULL;
> > > 
> > > I don't think this part is needed: once we COW devmap PMD entry, we split
> > > it into PTE table, so IIUC we never get here with PMD.
> > 
> > Hi Kirill,
> > 
> > Would you mind double-checking that?  You certainly know devmap
> > better than me, but I feel safer with Keno's original as above.
> > 
> > I can see that fs/dax.c dax_iomap_pmd_fault() does
> > 
> > 	/* Fall back to PTEs if we're going to COW */
> > 	if (write && !(vma->vm_flags & VM_SHARED))
> > 		goto fallback;
> > 
> > But isn't there a case of O_RDWR fd, VM_SHARED PROT_READ mmap, and
> > FOLL_FORCE write to it, which does not COW (but relies on FOLL_COW)?
> 
> And now I think I'm wrong, but please double-check even so: I think that
> case gets ruled out by the !is_cow_mapping(vm_flags) check in mm/gup.c,
> where we used to have a WARN_ON_ONCE() for a while.

Right, !is_cow_mapping(vm_flags) will filter the case out.

Also there's no way we will get FOLL_COW set for file THP (dax or not): we
never return VM_FAULT_WRITE there.

-- 
 Kirill A. Shutemov