[PATCH 4.9 146/157] Revert "gup: document and work around "COW can break either way" issue"

[Greg Kroah-Hartman]

From: Ben Hutchings <ben@xxxxxxxxxxxxxxx>

This reverts commit 9bbd42e79720122334226afad9ddcac1c3e6d373, which
was commit 17839856fd588f4ab6b789f482ed3ffd7c403e1f upstream.  The
backport was incorrect and incomplete:

* It forced the write flag on in the generic __get_user_pages_fast(),
  whereas only get_user_pages_fast() was supposed to do that.
* It only fixed the generic RCU-based implementation used by arm,
  arm64, and powerpc.  Before Linux 4.13, several other architectures
  had their own implementations: mips, s390, sparc, sh, and x86.

This will be followed by a (hopefully) correct backport.

Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 mm/gup.c         |   48 ++++++++----------------------------------------
 mm/huge_memory.c |    7 ++++---
 2 files changed, 12 insertions(+), 43 deletions(-)

--- a/mm/gup.c
+++ b/mm/gup.c
@@ -61,22 +61,13 @@ static int follow_pfn_pte(struct vm_area
 }
 
 /*
- * 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);
+	return pte_write(pte) ||
+		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
 }
 
 static struct page *follow_page_pte(struct vm_area_struct *vma,
@@ -586,18 +577,12 @@ static long __get_user_pages(struct task
 			if (!vma || check_vma_flags(vma, gup_flags))
 				return i ? : -EFAULT;
 			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);
+						gup_flags);
 				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
@@ -1518,10 +1503,6 @@ static int gup_pud_range(pgd_t pgd, unsi
 /*
  * Like get_user_pages_fast() except it's IRQ-safe in that it won't fall back to
  * the regular GUP. It will only return non-negative values.
- *
- * Careful, careful! COW breaking can go either way, so a non-write
- * access can get ambiguous page results. If you call this function without
- * 'write' set, you'd better be sure that you're ok with that ambiguity.
  */
 int __get_user_pages_fast(unsigned long start, int nr_pages, int write,
 			  struct page **pages)
@@ -1551,12 +1532,6 @@ int __get_user_pages_fast(unsigned long
 	 *
 	 * We do not adopt an rcu_read_lock(.) here as we also want to
 	 * block IPIs that come from THPs splitting.
-	 *
-	 * NOTE! We allow read-only gup_fast() here, 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.
 	 */
 
 	local_irq_save(flags);
@@ -1567,22 +1542,15 @@ int __get_user_pages_fast(unsigned long
 		next = pgd_addr_end(addr, end);
 		if (pgd_none(pgd))
 			break;
-		/*
-		 * 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.
-		 */
 		if (unlikely(pgd_huge(pgd))) {
-			if (!gup_huge_pgd(pgd, pgdp, addr, next, 1,
+			if (!gup_huge_pgd(pgd, pgdp, addr, next, write,
 					  pages, &nr))
 				break;
 		} else if (unlikely(is_hugepd(__hugepd(pgd_val(pgd))))) {
 			if (!gup_huge_pd(__hugepd(pgd_val(pgd)), addr,
-					 PGDIR_SHIFT, next, 1, pages, &nr))
+					 PGDIR_SHIFT, next, write, pages, &nr))
 				break;
-		} else if (!gup_pud_range(pgd, addr, next, 1, pages, &nr))
+		} else if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
 			break;
 	} while (pgdp++, addr = next, addr != end);
 	local_irq_restore(flags);
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1135,12 +1135,13 @@ out_unlock:
 }
 
 /*
- * 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,