Merge branch 'master' into for-4.20/upstream-fixes

Pull in a merge commit that brought in 3b692c55e58d ("HID: asus: only
support backlight when it's not driven by WMI") so that fixup could be
applied on top of it.

1 files changed, 16 insertions, 4 deletions

diff --git a/mm/mremap.c b/mm/mremap.c
index a9617e72e6b7..7f9f9180e401 100644
--- a/mm/mremap.c
+++ b/mm/mremap.c
@@ -521,6 +521,7 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
 	unsigned long ret = -EINVAL;
 	unsigned long charged = 0;
 	bool locked = false;
+	bool downgraded = false;
 	struct vm_userfaultfd_ctx uf = NULL_VM_UFFD_CTX;
 	LIST_HEAD(uf_unmap_early);
 	LIST_HEAD(uf_unmap);
@@ -557,12 +558,20 @@ SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
 	/*
 	 * Always allow a shrinking remap: that just unmaps
 	 * the unnecessary pages..
-	 * do_munmap does all the needed commit accounting
+	 * __do_munmap does all the needed commit accounting, and
+	 * downgrades mmap_sem to read if so directed.
 	 */
 	if (old_len >= new_len) {
-		ret = do_munmap(mm, addr+new_len, old_len - new_len, &uf_unmap);
-		if (ret && old_len != new_len)
+		int retval;
+
+		retval = __do_munmap(mm, addr+new_len, old_len - new_len,
+				  &uf_unmap, true);
+		if (retval < 0 && old_len != new_len) {
+			ret = retval;
 			goto out;
+		/* Returning 1 indicates mmap_sem is downgraded to read. */
+		} else if (retval == 1)
+			downgraded = true;
 		ret = addr;
 		goto out;
 	}
@@ -627,7 +636,10 @@ out:
 		vm_unacct_memory(charged);
 		locked = 0;
 	}
-	up_write(&current->mm->mmap_sem);
+	if (downgraded)
+		up_read(&current->mm->mmap_sem);
+	else
+		up_write(&current->mm->mmap_sem);
 	if (locked && new_len > old_len)
 		mm_populate(new_addr + old_len, new_len - old_len);
 	userfaultfd_unmap_complete(mm, &uf_unmap_early);