mm: remove SWAP_AGAIN in ttu

In 2002, [1] introduced SWAP_AGAIN.  At that time, try_to_unmap_one used
spin_trylock(&mm->page_table_lock) so it's really easy to contend and
fail to hold a lock so SWAP_AGAIN to keep LRU status makes sense.

However, now we changed it to mutex-based lock and be able to block
without skip pte so there is few of small window to return SWAP_AGAIN so
remove SWAP_AGAIN and just return SWAP_FAIL.

[1] c48c43e, minimal rmap

Link: http://lkml.kernel.org/r/1489555493-14659-7-git-send-email-minchan@kernel.org
Signed-off-by: Minchan Kim <minchan@kernel.org>
Cc: Anshuman Khandual <khandual@linux.vnet.ibm.com>
Cc: Hillf Danton <hillf.zj@alibaba-inc.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/rmap.c

@@ -1504,13 +1504,10 @@ static int page_mapcount_is_zero(struct page *page)
  * Return values are:
  *
  * SWAP_SUCCESS	- we succeeded in removing all mappings
- * SWAP_AGAIN	- we missed a mapping, try again later
  * SWAP_FAIL	- the page is unswappable
  */
 int try_to_unmap(struct page *page, enum ttu_flags flags)
 {
-	int ret;
-
 	struct rmap_walk_control rwc = {
 		.rmap_one = try_to_unmap_one,
 		.arg = (void *)flags,
@@ -1530,13 +1527,11 @@ int try_to_unmap(struct page *page, enum ttu_flags flags)
 		rwc.invalid_vma = invalid_migration_vma;
 
 	if (flags & TTU_RMAP_LOCKED)
-		ret = rmap_walk_locked(page, &rwc);
+		rmap_walk_locked(page, &rwc);
 	else
-		ret = rmap_walk(page, &rwc);
+		rmap_walk(page, &rwc);
 
-	if (!page_mapcount(page))
-		ret = SWAP_SUCCESS;
-	return ret;
+	return !page_mapcount(page) ? SWAP_SUCCESS : SWAP_FAIL;
 }
 
 static int page_not_mapped(struct page *page)

mm/vmscan.c

@@ -1150,8 +1150,6 @@ static unsigned long shrink_page_list(struct list_head *page_list,
 			case SWAP_FAIL:
 				nr_unmap_fail++;
 				goto activate_locked;
-			case SWAP_AGAIN:
-				goto keep_locked;
 			case SWAP_SUCCESS:
 				; /* try to free the page below */
 			}