>> @@ -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. > > Maybe we should WARN_ONCE() if have FOLL_COW here.
Sounds good to me. As I said, I don't have a testcase for this code path, I just noticed the same pattern. Will send an updated patch with the WARN_ONCE there.
