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SubjectRe: [PATCH] mm: Respect FOLL_FORCE/FOLL_COW for thp
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.

Hugh

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