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SubjectRe: [PATCH] mm/z3fold.c: Fix race between migration and destruction
Hi Henry,

Den fre 9 aug. 2019 6:46 emHenry Burns <henryburns@google.com> skrev:
>
> 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.


Thanks for the fix. Would you please comment why adding
flush_workqueue() isn't enough?

~Vitaly
>
>
> Signed-off-by: Henry Burns <henryburns@google.com>
> ---
> mm/z3fold.c | 51 +++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 51 insertions(+)
>
> diff --git a/mm/z3fold.c b/mm/z3fold.c
> index 78447cecfffa..e136d97ce56e 100644
> --- a/mm/z3fold.c
> +++ b/mm/z3fold.c
> @@ -40,6 +40,7 @@
> #include <linux/workqueue.h>
> #include <linux/slab.h>
> #include <linux/spinlock.h>
> +#include <linux/wait.h>
> #include <linux/zpool.h>
>
> /*
> @@ -161,8 +162,10 @@ 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;
> + int isolated_pages;
> };
>
> /*
> @@ -772,6 +775,7 @@ 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;
> @@ -811,6 +815,15 @@ 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_pages == 0;
> + spin_unlock(&pool->lock);
> + return ret;
> +}
> /**
> * z3fold_destroy_pool() - destroys an existing z3fold pool
> * @pool: the z3fold pool to be destroyed
> @@ -821,6 +834,13 @@ static void z3fold_destroy_pool(struct z3fold_pool *pool)
> {
> kmem_cache_destroy(pool->c_handle);
>
> + /*
> + * 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,
> * as any pending work on pool->compact_wq will call
> @@ -1317,6 +1337,28 @@ 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_pages <= 0);
> + pool->isolated_pages--;
> +
> + /*
> + * If we have no more isolated pages, we have to see if
> + * z3fold_destroy_pool() is waiting for a signal.
> + */
> + if (pool->isolated_pages == 0 && waitqueue_active(&pool->isolate_wait))
> + wake_up_all(&pool->isolate_wait);
> +}
> +
> +static void z3fold_inc_isolated(struct z3fold_pool *pool)
> +{
> + pool->isolated_pages++;
> +}
> +
> static bool z3fold_page_isolate(struct page *page, isolate_mode_t mode)
> {
> struct z3fold_header *zhdr;
> @@ -1343,6 +1385,7 @@ 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);
> + z3fold_inc_isolated(pool);
> spin_unlock(&pool->lock);
> z3fold_page_unlock(zhdr);
> return true;
> @@ -1417,6 +1460,10 @@ 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;
> @@ -1436,10 +1483,14 @@ 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);
> }
> --
> 2.22.0.770.g0f2c4a37fd-goog
>

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