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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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