Revert "mm/z3fold.c: fix race between migration and destruction"

With the original commit applied, z3fold_zpool_destroy() may get blocked
on wait_event() for indefinite time.  Revert this commit for the time
being to get rid of this problem since the issue the original commit
addresses is less severe.

Link: http://lkml.kernel.org/r/20190910123142.7a9c8d2de4d0acbc0977c602@gmail.com
Fixes: d776aaa9 ("mm/z3fold.c: fix race between migration and destruction")
Reported-by: Agustín Dall'Alba <agustin@dallalba.com.ar>
Signed-off-by: Vitaly Wool <vitalywool@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Vitaly Wool <vitalywool@gmail.com>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Jonathan Adams <jwadams@google.com>
Cc: Henry Burns <henrywolfeburns@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/z3fold.c

@@ -41,7 +41,6 @@
 #include <linux/workqueue.h>
 #include <linux/slab.h>
 #include <linux/spinlock.h>
-#include <linux/wait.h>
 #include <linux/zpool.h>
 #include <linux/magic.h>
 
@@ -146,8 +145,6 @@ struct z3fold_header {
  * @release_wq:	workqueue for safe page release
  * @work:	work_struct for safe page release
  * @inode:	inode for z3fold pseudo filesystem
- * @destroying: bool to stop migration once we start destruction
- * @isolated: int to count the number of pages currently in isolation
  *
  * This structure is allocated at pool creation time and maintains metadata
  * pertaining to a particular z3fold pool.
@@ -166,11 +163,8 @@ struct z3fold_pool {
 	const struct zpool_ops *zpool_ops;
 	struct workqueue_struct *compact_wq;
 	struct workqueue_struct *release_wq;
-	struct wait_queue_head isolate_wait;
 	struct work_struct work;
 	struct inode *inode;
-	bool destroying;
-	int isolated;
 };
 
 /*
@@ -775,7 +769,6 @@ static struct z3fold_pool *z3fold_create_pool(const char *name, gfp_t gfp,
 		goto out_c;
 	spin_lock_init(&pool->lock);
 	spin_lock_init(&pool->stale_lock);
-	init_waitqueue_head(&pool->isolate_wait);
 	pool->unbuddied = __alloc_percpu(sizeof(struct list_head)*NCHUNKS, 2);
 	if (!pool->unbuddied)
 		goto out_pool;
@@ -815,15 +808,6 @@ static struct z3fold_pool *z3fold_create_pool(const char *name, gfp_t gfp,
 	return NULL;
 }
 
-static bool pool_isolated_are_drained(struct z3fold_pool *pool)
-{
-	bool ret;
-
-	spin_lock(&pool->lock);
-	ret = pool->isolated == 0;
-	spin_unlock(&pool->lock);
-	return ret;
-}
 /**
  * z3fold_destroy_pool() - destroys an existing z3fold pool
  * @pool:	the z3fold pool to be destroyed
@@ -833,22 +817,6 @@ static bool pool_isolated_are_drained(struct z3fold_pool *pool)
 static void z3fold_destroy_pool(struct z3fold_pool *pool)
 {
 	kmem_cache_destroy(pool->c_handle);
-	/*
-	 * We set pool-> destroying under lock to ensure that
-	 * z3fold_page_isolate() sees any changes to destroying. This way we
-	 * avoid the need for any memory barriers.
-	 */
-
-	spin_lock(&pool->lock);
-	pool->destroying = true;
-	spin_unlock(&pool->lock);
-
-	/*
-	 * We need to ensure that no pages are being migrated while we destroy
-	 * these workqueues, as migration can queue work on either of the
-	 * workqueues.
-	 */
-	wait_event(pool->isolate_wait, !pool_isolated_are_drained(pool));
 
 	/*
 	 * We need to destroy pool->compact_wq before pool->release_wq,
@@ -1339,28 +1307,6 @@ static u64 z3fold_get_pool_size(struct z3fold_pool *pool)
 	return atomic64_read(&pool->pages_nr);
 }
 
-/*
- * z3fold_dec_isolated() expects to be called while pool->lock is held.
- */
-static void z3fold_dec_isolated(struct z3fold_pool *pool)
-{
-	assert_spin_locked(&pool->lock);
-	VM_BUG_ON(pool->isolated <= 0);
-	pool->isolated--;
-
-	/*
-	 * If we have no more isolated pages, we have to see if
-	 * z3fold_destroy_pool() is waiting for a signal.
-	 */
-	if (pool->isolated == 0 && waitqueue_active(&pool->isolate_wait))
-		wake_up_all(&pool->isolate_wait);
-}
-
-static void z3fold_inc_isolated(struct z3fold_pool *pool)
-{
-	pool->isolated++;
-}
-
 static bool z3fold_page_isolate(struct page *page, isolate_mode_t mode)
 {
 	struct z3fold_header *zhdr;
@@ -1387,34 +1333,6 @@ static bool z3fold_page_isolate(struct page *page, isolate_mode_t mode)
 		spin_lock(&pool->lock);
 		if (!list_empty(&page->lru))
 			list_del(&page->lru);
-		/*
-		 * We need to check for destruction while holding pool->lock, as
-		 * otherwise destruction could see 0 isolated pages, and
-		 * proceed.
-		 */
-		if (unlikely(pool->destroying)) {
-			spin_unlock(&pool->lock);
-			/*
-			 * If this page isn't stale, somebody else holds a
-			 * reference to it. Let't drop our refcount so that they
-			 * can call the release logic.
-			 */
-			if (unlikely(kref_put(&zhdr->refcount,
-					      release_z3fold_page_locked))) {
-				/*
-				 * If we get here we have kref problems, so we
-				 * should freak out.
-				 */
-				WARN(1, "Z3fold is experiencing kref problems\n");
-				z3fold_page_unlock(zhdr);
-				return false;
-			}
-			z3fold_page_unlock(zhdr);
-			return false;
-		}
-
-
-		z3fold_inc_isolated(pool);
 		spin_unlock(&pool->lock);
 		z3fold_page_unlock(zhdr);
 		return true;
@@ -1483,10 +1401,6 @@ static int z3fold_page_migrate(struct address_space *mapping, struct page *newpa
 
 	queue_work_on(new_zhdr->cpu, pool->compact_wq, &new_zhdr->work);
 
-	spin_lock(&pool->lock);
-	z3fold_dec_isolated(pool);
-	spin_unlock(&pool->lock);
-
 	page_mapcount_reset(page);
 	put_page(page);
 	return 0;
@@ -1506,14 +1420,10 @@ static void z3fold_page_putback(struct page *page)
 	INIT_LIST_HEAD(&page->lru);
 	if (kref_put(&zhdr->refcount, release_z3fold_page_locked)) {
 		atomic64_dec(&pool->pages_nr);
-		spin_lock(&pool->lock);
-		z3fold_dec_isolated(pool);
-		spin_unlock(&pool->lock);
 		return;
 	}
 	spin_lock(&pool->lock);
 	list_add(&page->lru, &pool->lru);
-	z3fold_dec_isolated(pool);
 	spin_unlock(&pool->lock);
 	z3fold_page_unlock(zhdr);
 }