[10/11] mm/zsmalloc.c: fix race condition in zs_destroy_pool

Commit Message

Andrew Morton Aug. 25, 2019, 12:55 a.m. UTC

From: Henry Burns <[e-mail]>
Subject: mm/zsmalloc.c: fix race condition in zs_destroy_pool

In zs_destroy_pool() we call flush_work(&pool->free_work).  However, we
have no guarantee that migration isn't happening in the background at that
time.

Since migration can't directly free pages, it relies on free_work being
scheduled to free the pages.  But there's nothing preventing an
in-progress migrate from queuing the work *after*
zs_unregister_migration() has called flush_work().  Which would mean pages
still pointing at the inode when we free it.

Since we know at destroy time all objects should be free, no new
migrations can come in (since zs_page_isolate() fails for fully-free
zspages).  This means it is sufficient to track a "# isolated zspages"
count by class, and have the destroy logic ensure all such pages have
drained before proceeding.  Keeping that state under the class spinlock
keeps the logic straightforward.

In this case a memory leak could lead to an eventual crash if
compaction hits the leaked page.  This crash would only occur if people
are changing their zswap backend at runtime (which eventually starts
destruction).

Link: http://lkml.kernel.org/r/[e-mail]
Fixes: 48b4800a1c6a ("zsmalloc: page migration support")
Signed-off-by: Henry Burns <[e-mail]>
Reviewed-by: Sergey Senozhatsky <[e-mail]>
Cc: Henry Burns <[e-mail]>
Cc: Minchan Kim <[e-mail]>
Cc: Shakeel Butt <[e-mail]>
Cc: Jonathan Adams <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 mm/zsmalloc.c |   61 ++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 59 insertions(+), 2 deletions(-)

Patch

--- a/mm/zsmalloc.c~mm-zsmallocc-fix-race-condition-in-zs_destroy_pool
+++ a/mm/zsmalloc.c
@@ -54,6 +54,7 @@ 
 #include <linux/mount.h>
 #include <linux/pseudo_fs.h>
 #include <linux/migrate.h>
+#include <linux/wait.h>
 #include <linux/pagemap.h>
 #include <linux/fs.h>
 
@@ -268,6 +269,10 @@  struct zs_pool {
 #ifdef CONFIG_COMPACTION
 	struct inode *inode;
 	struct work_struct free_work;
+	/* A wait queue for when migration races with async_free_zspage() */
+	struct wait_queue_head migration_wait;
+	atomic_long_t isolated_pages;
+	bool destroying;
 #endif
 };
 
@@ -1874,6 +1879,19 @@  static void putback_zspage_deferred(stru
 
 }
 
+static inline void zs_pool_dec_isolated(struct zs_pool *pool)
+{
+	VM_BUG_ON(atomic_long_read(&pool->isolated_pages) <= 0);
+	atomic_long_dec(&pool->isolated_pages);
+	/*
+	 * There's no possibility of racing, since wait_for_isolated_drain()
+	 * checks the isolated count under &class->lock after enqueuing
+	 * on migration_wait.
+	 */
+	if (atomic_long_read(&pool->isolated_pages) == 0 && pool->destroying)
+		wake_up_all(&pool->migration_wait);
+}
+
 static void replace_sub_page(struct size_class *class, struct zspage *zspage,
 				struct page *newpage, struct page *oldpage)
 {
@@ -1943,6 +1961,7 @@  static bool zs_page_isolate(struct page
 	 */
 	if (!list_empty(&zspage->list) && !is_zspage_isolated(zspage)) {
 		get_zspage_mapping(zspage, &class_idx, &fullness);
+		atomic_long_inc(&pool->isolated_pages);
 		remove_zspage(class, zspage, fullness);
 	}
 
@@ -2042,8 +2061,16 @@  static int zs_page_migrate(struct addres
 	 * Page migration is done so let's putback isolated zspage to
 	 * the list if @page is final isolated subpage in the zspage.
 	 */
-	if (!is_zspage_isolated(zspage))
+	if (!is_zspage_isolated(zspage)) {
+		/*
+		 * We cannot race with zs_destroy_pool() here because we wait
+		 * for isolation to hit zero before we start destroying.
+		 * Also, we ensure that everyone can see pool->destroying before
+		 * we start waiting.
+		 */
 		putback_zspage_deferred(pool, class, zspage);
+		zs_pool_dec_isolated(pool);
+	}
 
 	reset_page(page);
 	put_page(page);
@@ -2094,8 +2121,8 @@  static void zs_page_putback(struct page
 		 * so let's defer.
 		 */
 		putback_zspage_deferred(pool, class, zspage);
+		zs_pool_dec_isolated(pool);
 	}
-
 	spin_unlock(&class->lock);
 }
 
@@ -2118,8 +2145,36 @@  static int zs_register_migration(struct
 	return 0;
 }
 
+static bool pool_isolated_are_drained(struct zs_pool *pool)
+{
+	return atomic_long_read(&pool->isolated_pages) == 0;
+}
+
+/* Function for resolving migration */
+static void wait_for_isolated_drain(struct zs_pool *pool)
+{
+
+	/*
+	 * We're in the process of destroying the pool, so there are no
+	 * active allocations. zs_page_isolate() fails for completely free
+	 * zspages, so we need only wait for the zs_pool's isolated
+	 * count to hit zero.
+	 */
+	wait_event(pool->migration_wait,
+		   pool_isolated_are_drained(pool));
+}
+
 static void zs_unregister_migration(struct zs_pool *pool)
 {
+	pool->destroying = true;
+	/*
+	 * We need a memory barrier here to ensure global visibility of
+	 * pool->destroying. Thus pool->isolated pages will either be 0 in which
+	 * case we don't care, or it will be > 0 and pool->destroying will
+	 * ensure that we wake up once isolation hits 0.
+	 */
+	smp_mb();
+	wait_for_isolated_drain(pool); /* This can block */
 	flush_work(&pool->free_work);
 	iput(pool->inode);
 }
@@ -2357,6 +2412,8 @@  struct zs_pool *zs_create_pool(const cha
 	if (!pool->name)
 		goto err;
 
+	init_waitqueue_head(&pool->migration_wait);
+
 	if (create_cache(pool))
 		goto err;