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
