mm/gup: Remove enfornced COW mechanism

With the more strict (but greatly simplified) page reuse logic in
do_wp_page(), we can safely go back to the world where cow is not
enforced with writes.

This essentially reverts commit 17839856 ("gup: document and work
around 'COW can break either way' issue").  There are some context
differences due to some changes later on around it:

  2170ecfa ("drm/i915: convert get_user_pages() --> pin_user_pages()", 2020-06-03)
  376a34ef ("mm/gup: refactor and de-duplicate gup_fast() code", 2020-06-03)

Some lines moved back and forth with those, but this revert patch should
have striped out and covered all the enforced cow bits anyways.
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/gpu/drm/i915/gem/i915_gem_userptr.c

@@ -599,14 +599,6 @@ static int i915_gem_userptr_get_pages(struct drm_i915_gem_object *obj)
 				      GFP_KERNEL |
 				      __GFP_NORETRY |
 				      __GFP_NOWARN);
-		/*
-		 * Using __get_user_pages_fast() with a read-only
-		 * access is questionable. A read-only page may be
-		 * COW-broken, and then this might end up giving
-		 * the wrong side of the COW..
-		 *
-		 * We may or may not care.
-		 */
 		if (pvec) {
 			/* defer to worker if malloc fails */
 			if (!i915_gem_object_is_readonly(obj))

mm/gup.c

@@ -381,22 +381,13 @@ static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
 }
 
 /*
- * FOLL_FORCE or a forced COW break can write even to unwritable pte's,
- * but only after we've gone through a COW cycle and they are dirty.
+ * FOLL_FORCE can write to even unwritable pte's, but only
+ * after we've gone through a COW cycle and they are dirty.
  */
 static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
 {
-	return pte_write(pte) || ((flags & FOLL_COW) && pte_dirty(pte));
-}
-
-/*
- * A (separate) COW fault might break the page the other way and
- * get_user_pages() would return the page from what is now the wrong
- * VM. So we need to force a COW break at GUP time even for reads.
- */
-static inline bool should_force_cow_break(struct vm_area_struct *vma, unsigned int flags)
-{
-	return is_cow_mapping(vma->vm_flags) && (flags & (FOLL_GET | FOLL_PIN));
+	return pte_write(pte) ||
+		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
 }
 
 static struct page *follow_page_pte(struct vm_area_struct *vma,
@@ -1075,11 +1066,9 @@ static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
 				goto out;
 			}
 			if (is_vm_hugetlb_page(vma)) {
-				if (should_force_cow_break(vma, foll_flags))
-					foll_flags |= FOLL_WRITE;
 				i = follow_hugetlb_page(mm, vma, pages, vmas,
 						&start, &nr_pages, i,
-						foll_flags, locked);
+						gup_flags, locked);
 				if (locked && *locked == 0) {
 					/*
 					 * We've got a VM_FAULT_RETRY
@@ -1093,10 +1082,6 @@ static long __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
 				continue;
 			}
 		}
-
-		if (should_force_cow_break(vma, foll_flags))
-			foll_flags |= FOLL_WRITE;
-
 retry:
 		/*
 		 * If we have a pending SIGKILL, don't keep faulting pages and
@@ -2763,19 +2748,6 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
 		return -EFAULT;
 
 	/*
-	 * The FAST_GUP case requires FOLL_WRITE even for pure reads,
-	 * because get_user_pages() may need to cause an early COW in
-	 * order to avoid confusing the normal COW routines. So only
-	 * targets that are already writable are safe to do by just
-	 * looking at the page tables.
-	 *
-	 * NOTE! With FOLL_FAST_ONLY we allow read-only gup_fast() here,
-	 * because there is no slow path to fall back on. But you'd
-	 * better be careful about possible COW pages - you'll get _a_
-	 * COW page, but not necessarily the one you intended to get
-	 * depending on what COW event happens after this. COW may break
-	 * the page copy in a random direction.
-	 *
 	 * Disable interrupts. The nested form is used, in order to allow
 	 * full, general purpose use of this routine.
 	 *
@@ -2788,8 +2760,6 @@ static int internal_get_user_pages_fast(unsigned long start, int nr_pages,
 	 */
 	if (IS_ENABLED(CONFIG_HAVE_FAST_GUP) && gup_fast_permitted(start, end)) {
 		unsigned long fast_flags = gup_flags;
-		if (!(gup_flags & FOLL_FAST_ONLY))
-			fast_flags |= FOLL_WRITE;
 
 		local_irq_save(flags);
 		gup_pgd_range(addr, end, fast_flags, pages, &nr_pinned);

mm/huge_memory.c

@@ -1312,12 +1312,13 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf, pmd_t orig_pmd)
 }
 
 /*
- * FOLL_FORCE or a forced COW break can write even to unwritable pmd's,
- * but only after we've gone through a COW cycle and they are dirty.
+ * 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_COW) && pmd_dirty(pmd));
+	return pmd_write(pmd) ||
+	       ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pmd_dirty(pmd));
 }
 
 struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,