From: Henry Burns henryburns@google.com Subject: mm/z3fold.c: fix race between migration and destruction
In z3fold_destroy_pool() we call destroy_workqueue(&pool->compact_wq). However, we have no guarantee that migration isn't happening in the background at that time.
Migration directly calls queue_work_on(pool->compact_wq), if destruction wins that race we are using a destroyed workqueue.
Link: http://lkml.kernel.org/r/20190809213828.202833-1-henryburns@google.com Signed-off-by: Henry Burns henryburns@google.com 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 ---
mm/z3fold.c | 89 ++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 89 insertions(+)
--- a/mm/z3fold.c~mm-z3foldc-fix-race-between-migration-and-destruction +++ a/mm/z3fold.c @@ -41,6 +41,7 @@ #include <linux/workqueue.h> #include <linux/slab.h> #include <linux/spinlock.h> +#include <linux/wait.h> #include <linux/zpool.h> #include <linux/magic.h>
@@ -145,6 +146,8 @@ 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. @@ -163,8 +166,11 @@ 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; };
/* @@ -769,6 +775,7 @@ static struct z3fold_pool *z3fold_create 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; @@ -808,6 +815,15 @@ out: 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 @@ -817,6 +833,22 @@ out: 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, @@ -1307,6 +1339,28 @@ static u64 z3fold_get_pool_size(struct z 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; @@ -1333,6 +1387,33 @@ static bool z3fold_page_isolate(struct p 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"); + return false; + } + z3fold_page_unlock(zhdr); + return false; + } + + + z3fold_inc_isolated(pool); spin_unlock(&pool->lock); z3fold_page_unlock(zhdr); return true; @@ -1401,6 +1482,10 @@ static int z3fold_page_migrate(struct ad
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; @@ -1420,10 +1505,14 @@ static void z3fold_page_putback(struct p 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); } _