[PATCH v9 08/20] mm/lru: add page isolation precondition in __isolate_lru_page

[Alex Shi]

Johannes Weiner has suggested:
"
So here is a crazy idea that may be worth exploring:

Right now, pgdat->lru_lock protects both PageLRU *and* the lruvec's
linked list.

Can we make PageLRU atomic and use it to stabilize the lru_lock
instead, and then use the lru_lock only serialize list operations?
...
"

Yes, this patch is doing so on  __isolate_lru_page which is the core
page isolation func in compaction and shrinking path.

This patch move clear page lru action before compaction getting lru_lock,
makes it as a necessary condition for page isolation. Hence, PageLRU may
be cleared druing shrink_inactive_list path for isolation reason. If so,
we can skip that page's in reclaim.

It's a preparation for later per memcg lru_lock change.

Suggested-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Signed-off-by: Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx
---
 include/linux/swap.h |  2 +-
 mm/compaction.c      | 25 +++++++++++++++++--------
 mm/vmscan.c          | 48 ++++++++++++++++++++++++++----------------------
 3 files changed, 44 insertions(+), 31 deletions(-)

diff --git a/include/linux/swap.h b/include/linux/swap.h
index c555e8f161ad..69f0794f1da3 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -351,7 +351,7 @@ extern void lru_cache_add_active_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(struct page *page, isolate_mode_t mode);
+extern int __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,
diff --git a/mm/compaction.c b/mm/compaction.c
index 672d3c78c6ab..1baba328d089 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -948,6 +948,23 @@ static bool too_many_isolated(pg_data_t *pgdat)
 		if (!(cc->gfp_mask & __GFP_FS) && page_mapping(page))
 			goto isolate_fail;
 
+		if (__isolate_lru_page_prepare(page, isolate_mode) != 0)
+			goto isolate_fail;
+
+		/*
+		 * 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 isolate_fail;
+
+		/* Try isolate the page */
+		if (!TestClearPageLRU(page)) {
+			put_page(page);
+			goto isolate_fail;
+		}
+
 		/* If we already hold the lock, we can skip some rechecking */
 		if (!locked) {
 			locked = compact_lock_irqsave(&pgdat->lru_lock,
@@ -960,10 +977,6 @@ static bool too_many_isolated(pg_data_t *pgdat)
 					goto isolate_abort;
 			}
 
-			/* Recheck PageLRU and PageCompound under lock */
-			if (!PageLRU(page))
-				goto isolate_fail;
-
 			/*
 			 * Page become compound since the non-locked check,
 			 * and it's on LRU. It can only be a THP so the order
@@ -977,10 +990,6 @@ static bool too_many_isolated(pg_data_t *pgdat)
 
 		lruvec = mem_cgroup_page_lruvec(page, pgdat);
 
-		/* Try isolate the page */
-		if (__isolate_lru_page(page, isolate_mode) != 0)
-			goto isolate_fail;
-
 		VM_BUG_ON_PAGE(PageCompound(page), page);
 
 		/* Successfully isolated */
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 8958454d50fe..bc2ec3fe4f48 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -1522,20 +1522,20 @@ unsigned long reclaim_clean_pages_from_list(struct zone *zone,
  *
  * returns 0 on success, -ve errno on failure.
  */
-int __isolate_lru_page(struct page *page, isolate_mode_t mode)
+int __isolate_lru_page_prepare(struct page *page, isolate_mode_t mode)
 {
 	int ret = -EINVAL;
 
-	/* Only take pages on the LRU. */
-	if (!PageLRU(page))
-		return ret;
-
 	/* Compaction should not handle unevictable pages but CMA can do so */
 	if (PageUnevictable(page) && !(mode & ISOLATE_UNEVICTABLE))
 		return ret;
 
 	ret = -EBUSY;
 
+	/* Only take pages on the LRU. */
+	if (!PageLRU(page))
+		return ret;
+
 	/*
 	 * To minimise LRU disruption, the caller can indicate that it only
 	 * wants to isolate pages it will be able to operate on without
@@ -1576,20 +1576,9 @@ int __isolate_lru_page(struct page *page, isolate_mode_t mode)
 	if ((mode & ISOLATE_UNMAPPED) && page_mapped(page))
 		return ret;
 
-	if (likely(get_page_unless_zero(page))) {
-		/*
-		 * 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.
-		 */
-		ClearPageLRU(page);
-		ret = 0;
-	}
-
-	return ret;
+	return 0;
 }
 
-
 /*
  * Update LRU sizes after isolating pages. The LRU size updates must
  * be complete before mem_cgroup_update_lru_size due to a santity check.
@@ -1653,8 +1642,6 @@ static unsigned long isolate_lru_pages(unsigned long nr_to_scan,
 		page = lru_to_page(src);
 		prefetchw_prev_lru_page(page, src, flags);
 
-		VM_BUG_ON_PAGE(!PageLRU(page), page);
-
 		nr_pages = compound_nr(page);
 		total_scan += nr_pages;
 
@@ -1675,17 +1662,34 @@ 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(page, mode)) {
+		switch (__isolate_lru_page_prepare(page, mode)) {
 		case 0:
+			/*
+			 * 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 (!TestClearPageLRU(page)) {
+				/*
+				 * This page may in other isolation path,
+				 * but we still hold lru_lock.
+				 */
+				put_page(page);
+				goto busy;
+			}
+
 			nr_taken += nr_pages;
 			nr_zone_taken[page_zonenum(page)] += nr_pages;
 			list_move(&page->lru, dst);
 			break;
-
+busy:
 		case -EBUSY:
 			/* else it is being freed elsewhere */
 			list_move(&page->lru, src);
-			continue;
+			break;
 
 		default:
 			BUG();
-- 
1.8.3.1