Merge tag 'media/v4.10-2' of git://git.kernel.org/pub/scm/linux/kernel/git/mchehab...

[mirror_ubuntu-zesty-kernel.git] / mm / huge_memory.c

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 9a6bd6c8d55a6691047e516a46c2cf6b931b912d..5f3ad65c85de01fa6e4c8a07ef9494410bf2b133 100644 (file)
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -783,6 +783,12 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
 
        assert_spin_locked(pmd_lockptr(mm, pmd));
 
+       /*
+        * When we COW a devmap PMD entry, we split it into PTEs, so we should
+        * not be in this function with `flags & FOLL_COW` set.
+        */
+       WARN_ONCE(flags & FOLL_COW, "mm: In follow_devmap_pmd with FOLL_COW set");
+
        if (flags & FOLL_WRITE && !pmd_write(*pmd))
                return NULL;
 
@@ -1128,6 +1134,16 @@ out_unlock:
        return ret;
 }
 
+/*
+ * FOLL_FORCE can write to even unwritable pmd's, but only
+ * after we've gone through a COW cycle and they are dirty.
+ */
+static inline bool can_follow_write_pmd(pmd_t pmd, unsigned int flags)
+{
+       return pmd_write(pmd) ||
+              ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pmd_dirty(pmd));
+}
+
 struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
                                   unsigned long addr,
                                   pmd_t *pmd,
@@ -1138,7 +1154,7 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
 
        assert_spin_locked(pmd_lockptr(mm, pmd));
 
-       if (flags & FOLL_WRITE && !pmd_write(*pmd))
+       if (flags & FOLL_WRITE && !can_follow_write_pmd(*pmd, flags))
                goto out;
 
        /* Avoid dumping huge zero page */