Date:   Tue, 10 Sep 2019 12:31:42 +0200
From:   Vitaly Wool <vitalywool@...il.com>
To:     Linux-MM <linux-mm@...ck.org>, linux-kernel@...r.kernel.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Agustín Dall'Alba <agustin@...lalba.com.ar>,
        Henry Burns <henrywolfeburns@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: [PATCH] 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.

This reverts commit d776aaa9895eb6eb770908e899cb7f5bd5025b3c.

Reported-by: Agustín Dall'Alba <agustin@...lalba.com.ar>
Signed-off-by: Vitaly Wool <vitalywool@...il.com>
---
 mm/z3fold.c | 90 -----------------------------------------------------
 1 file changed, 90 deletions(-)

diff --git a/mm/z3fold.c b/mm/z3fold.c
index 75b7962439ff..ed19d98c9dcd 100644
--- a/mm/z3fold.c
+++ b/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);
 }
-- 
2.17.1

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