mm/vmscan: __isolate_lru_page_prepare() cleanup

The function just returns 2 results, so using a 'switch' to deal with its
result is unnecessary.  Also simplify it to a bool func as Vlastimil
suggested.

Also remove 'goto' by reusing list_move(), and take Matthew Wilcox's
suggestion to update comments in function.

Link: https://lkml.kernel.org/r/728874d7-2d93-4049-68c1-dcc3b2d52ccd@linux.alibaba.comSigned-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Yu Zhao <yuzhao@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/swap.h

@@ -356,7 +356,7 @@ extern void lru_cache_add_inactive_or_unevictable(struct page *page,
 extern unsigned long zone_reclaimable_pages(struct zone *zone);
 extern unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
 					gfp_t gfp_mask, nodemask_t *mask);
-extern int __isolate_lru_page_prepare(struct page *page, isolate_mode_t mode);
+extern bool __isolate_lru_page_prepare(struct page *page, isolate_mode_t mode);
 extern unsigned long try_to_free_mem_cgroup_pages(struct mem_cgroup *memcg,
 						  unsigned long nr_pages,
 						  gfp_t gfp_mask,

mm/compaction.c

@@ -988,7 +988,7 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
 		if (unlikely(!get_page_unless_zero(page)))
 			goto isolate_fail;
 
-		if (__isolate_lru_page_prepare(page, isolate_mode) != 0)
+		if (!__isolate_lru_page_prepare(page, isolate_mode))
 			goto isolate_fail_put;
 
 		/* Try isolate the page */

mm/vmscan.c

@@ -1539,19 +1539,17 @@ unsigned int reclaim_clean_pages_from_list(struct zone *zone,
  * page:	page to consider
  * mode:	one of the LRU isolation modes defined above
  *
- * returns 0 on success, -ve errno on failure.
+ * returns true on success, false on failure.
  */
-int __isolate_lru_page_prepare(struct page *page, isolate_mode_t mode)
+bool __isolate_lru_page_prepare(struct page *page, isolate_mode_t mode)
 {
-	int ret = -EBUSY;
-
 	/* Only take pages on the LRU. */
 	if (!PageLRU(page))
-		return ret;
+		return false;
 
 	/* Compaction should not handle unevictable pages but CMA can do so */
 	if (PageUnevictable(page) && !(mode & ISOLATE_UNEVICTABLE))
-		return ret;
+		return false;
 
 	/*
 	 * To minimise LRU disruption, the caller can indicate that it only
@@ -1564,7 +1562,7 @@ int __isolate_lru_page_prepare(struct page *page, isolate_mode_t mode)
 	if (mode & ISOLATE_ASYNC_MIGRATE) {
 		/* All the caller can do on PageWriteback is block */
 		if (PageWriteback(page))
-			return ret;
+			return false;
 
 		if (PageDirty(page)) {
 			struct address_space *mapping;
@@ -1580,20 +1578,20 @@ int __isolate_lru_page_prepare(struct page *page, isolate_mode_t mode)
 			 * from the page cache.
 			 */
 			if (!trylock_page(page))
-				return ret;
+				return false;
 
 			mapping = page_mapping(page);
 			migrate_dirty = !mapping || mapping->a_ops->migratepage;
 			unlock_page(page);
 			if (!migrate_dirty)
-				return ret;
+				return false;
 		}
 	}
 
 	if ((mode & ISOLATE_UNMAPPED) && page_mapped(page))
-		return ret;
+		return false;
 
-	return 0;
+	return true;
 }
 
 /*
@@ -1677,35 +1675,31 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
 		 * only when the page is being freed somewhere else.
 		 */
 		scan += nr_pages;
-		switch (__isolate_lru_page_prepare(page, mode)) {
-		case 0:
+		if (!__isolate_lru_page_prepare(page, mode)) {
+			/* It is being freed elsewhere */
+			list_move(&page->lru, src);
+			continue;
+		}
 		/*
 		 * Be careful not to clear PageLRU until after we're
 		 * sure the page is not being freed elsewhere -- the
 		 * page release code relies on it.
 		 */
-			if (unlikely(!get_page_unless_zero(page)))
-				goto busy;
+		if (unlikely(!get_page_unless_zero(page))) {
+			list_move(&page->lru, src);
+			continue;
+		}
 
 		if (!TestClearPageLRU(page)) {
-				/*
-				 * This page may in other isolation path,
-				 * but we still hold lru_lock.
-				 */
+			/* Another thread is already isolating this page */
 			put_page(page);
-				goto busy;
+			list_move(&page->lru, src);
+			continue;
 		}
 
 		nr_taken += nr_pages;
 		nr_zone_taken[page_zonenum(page)] += nr_pages;
 		list_move(&page->lru, dst);
-			break;
-
-		default:
-busy:
-			/* else it is being freed elsewhere */
-			list_move(&page->lru, src);
-		}
 	}
 
 	/*