[Devel] [PATCH v2] tcache: Repeat invalidation in tcache_invalidate_node_pages()

[Kirill Tkhai]

When there are more than 2 users of a page,  __tcache_page_tree_delete()
fails to freeze it. We skip it and never try to freeze it again.

In this case the page remains not invalidated, and tcache_node->nr_pages
never decremented. Later, we catch WARN_ON() reporting about this.

tcache_shrink_scan()                                   tcache_destroy_pool
   tcache_lru_isolate()                                
      tcache_grab_pool()
      ...
      page_cache_get_speculative() -->cnt == 2

      ...
      tcache_put_pool() --> pool cnt zero
      ...                                                  wait_for_completion(&pool->completion);
   tcache_reclaim_pages                                    tcache_invalidate_node_pages()
      __tcache_reclaim_page()                                  tcache_lookup()
                                                                  page_cache_get_speculative  --> cnt == 3
                                                               __tcache_page_tree_delete
        page_ref_freeze(2) -->fail                                page_ref_freeze(2) -->fail

The patch fixes the problem. In case of we failed to invalidate a page,
we remember this, and return to such pages after others are invalidated.

https://jira.sw.ru/browse/PSBM-78354

v2: Also fix tcache_detach_page()

Signed-off-by: Kirill Tkhai <ktkhai at virtuozzo.com>
---
 mm/tcache.c |   21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/mm/tcache.c b/mm/tcache.c
index d1a2c53e11a3..760e417d491b 100644
--- a/mm/tcache.c
+++ b/mm/tcache.c
@@ -850,6 +850,14 @@ static struct page *tcache_detach_page(struct tcache_node *node, pgoff_t index,
 		if (page)
 			tcache_lru_del(node->pool, page, reused);
 		local_irq_restore(flags);
+		/*
+		 * Shrinker could isolated the page in parallel
+		 * with us. This case page_ref_freeze(page, 2)
+		 * in __tcache_page_tree_delete() fails, and
+		 * we have to repeat the cycle.
+		 */
+		if (!page)
+			goto repeat;
 	}
 
 	return page;
@@ -903,13 +911,15 @@ tcache_invalidate_node_pages(struct tcache_node *node)
 	struct page *pages[TCACHE_PAGEVEC_SIZE];
 	pgoff_t index = 0;
 	unsigned nr_pages;
+	bool repeat;
 	int i;
 
 	/*
 	 * First forbid new page insertions - see tcache_page_tree_replace.
 	 */
 	node->invalidated = true;
-
+again:
+	repeat = false;
 	while ((nr_pages = tcache_lookup(pages, node, index,
 						TCACHE_PAGEVEC_SIZE, indices))) {
 		for (i = 0; i < nr_pages; i++) {
@@ -925,13 +935,20 @@ tcache_invalidate_node_pages(struct tcache_node *node)
 				tcache_lru_del(node->pool, page, false);
 				local_irq_enable();
 				tcache_put_page(page);
-			} else
+			} else {
 				local_irq_enable();
+				repeat = true;
+			}
 		}
 		cond_resched();
 		index++;
 	}
 
+	if (repeat) {
+		index = 0;
+		goto again;
+	}
+
 	WARN_ON(node->nr_pages != 0);
 }