1 files changed, 43 insertions, 21 deletions

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 8a7c1b344abe..e9414ee57c5b 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1040,12 +1040,6 @@ 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");
-
 	/* FOLL_GET and FOLL_PIN are mutually exclusive. */
 	if (WARN_ON_ONCE((flags & (FOLL_PIN | FOLL_GET)) ==
 			 (FOLL_PIN | FOLL_GET)))
@@ -1395,14 +1389,42 @@ fallback:
 	return VM_FAULT_FALLBACK;
 }
 
-/*
- * 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)
+/* FOLL_FORCE can write to even unwritable PMDs in COW mappings. */
+static inline bool can_follow_write_pmd(pmd_t pmd, struct page *page,
+					struct vm_area_struct *vma,
+					unsigned int flags)
 {
-	return pmd_write(pmd) ||
-	       ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pmd_dirty(pmd));
+	/* If the pmd is writable, we can write to the page. */
+	if (pmd_write(pmd))
+		return true;
+
+	/* Maybe FOLL_FORCE is set to override it? */
+	if (!(flags & FOLL_FORCE))
+		return false;
+
+	/* But FOLL_FORCE has no effect on shared mappings */
+	if (vma->vm_flags & (VM_MAYSHARE | VM_SHARED))
+		return false;
+
+	/* ... or read-only private ones */
+	if (!(vma->vm_flags & VM_MAYWRITE))
+		return false;
+
+	/* ... or already writable ones that just need to take a write fault */
+	if (vma->vm_flags & VM_WRITE)
+		return false;
+
+	/*
+	 * See can_change_pte_writable(): we broke COW and could map the page
+	 * writable if we have an exclusive anonymous page ...
+	 */
+	if (!page || !PageAnon(page) || !PageAnonExclusive(page))
+		return false;
+
+	/* ... and a write-fault isn't required for other reasons. */
+	if (vma_soft_dirty_enabled(vma) && !pmd_soft_dirty(pmd))
+		return false;
+	return !userfaultfd_huge_pmd_wp(vma, pmd);
 }
 
 struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
@@ -1411,12 +1433,16 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
 				   unsigned int flags)
 {
 	struct mm_struct *mm = vma->vm_mm;
-	struct page *page = NULL;
+	struct page *page;
 
 	assert_spin_locked(pmd_lockptr(mm, pmd));
 
-	if (flags & FOLL_WRITE && !can_follow_write_pmd(*pmd, flags))
-		goto out;
+	page = pmd_page(*pmd);
+	VM_BUG_ON_PAGE(!PageHead(page) && !is_zone_device_page(page), page);
+
+	if ((flags & FOLL_WRITE) &&
+	    !can_follow_write_pmd(*pmd, page, vma, flags))
+		return NULL;
 
 	/* Avoid dumping huge zero page */
 	if ((flags & FOLL_DUMP) && is_huge_zero_pmd(*pmd))
@@ -1424,10 +1450,7 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
 
 	/* Full NUMA hinting faults to serialise migration in fault paths */
 	if ((flags & FOLL_NUMA) && pmd_protnone(*pmd))
-		goto out;
-
-	page = pmd_page(*pmd);
-	VM_BUG_ON_PAGE(!PageHead(page) && !is_zone_device_page(page), page);
+		return NULL;
 
 	if (!pmd_write(*pmd) && gup_must_unshare(flags, page))
 		return ERR_PTR(-EMLINK);
@@ -1444,7 +1467,6 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
 	page += (addr & ~HPAGE_PMD_MASK) >> PAGE_SHIFT;
 	VM_BUG_ON_PAGE(!PageCompound(page) && !is_zone_device_page(page), page);
 
-out:
 	return page;
 }