[PATCH 3/4] mm: swapoff: take notice of completion sooner

[Hugh Dickins <hughd@google.com>]

The old try_to_unuse() implementation was driven by find_next_to_unuse(),
which terminated as soon as all the swap had been freed.  Add inuse_pages
checks now (alongside signal_pending()) to stop scanning mms and swap_map
once finished.  The same ought to be done in shmem_unuse() too, but never
was before, and needs a different interface: so leave it as is for now.

Fixes: b56a2d8af914 ("mm: rid swapoff of quadratic complexity")
Signed-off-by: Hugh Dickins <hughd@google.com>
---

 mm/swapfile.c |   19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

--- 5.1-rc4/mm/swapfile.c	2019-04-07 19:15:01.269054187 -0700
+++ linux/mm/swapfile.c	2019-04-07 19:17:13.291957539 -0700
@@ -2051,11 +2051,9 @@ retry:
 
 	spin_lock(&mmlist_lock);
 	p = &init_mm.mmlist;
-	while ((p = p->next) != &init_mm.mmlist) {
-		if (signal_pending(current)) {
-			retval = -EINTR;
-			break;
-		}
+	while (si->inuse_pages &&
+	       !signal_pending(current) &&
+	       (p = p->next) != &init_mm.mmlist) {
 
 		mm = list_entry(p, struct mm_struct, mmlist);
 		if (!mmget_not_zero(mm))
@@ -2082,7 +2080,9 @@ retry:
 	mmput(prev_mm);
 
 	i = 0;
-	while ((i = find_next_to_unuse(si, i, frontswap)) != 0) {
+	while (si->inuse_pages &&
+	       !signal_pending(current) &&
+	       (i = find_next_to_unuse(si, i, frontswap)) != 0) {
 
 		entry = swp_entry(type, i);
 		page = find_get_page(swap_address_space(entry), i);
@@ -2123,8 +2123,11 @@ retry:
 	 * separate lists, and wait for those lists to be emptied; but it's
 	 * easier and more robust (though cpu-intensive) just to keep retrying.
 	 */
-	if (si->inuse_pages)
-		goto retry;
+	if (si->inuse_pages) {
+		if (!signal_pending(current))
+			goto retry;
+		retval = -EINTR;
+	}
 out:
 	return (retval == FRONTSWAP_PAGES_UNUSED) ? 0 : retval;
 }