Re: [PATCH 2/2] mm/zsmalloc.c: Fix race condition in zs_destroy_pool

[Minchan Kim]

Hi Henry,

On Thu, Aug 01, 2019 at 06:53:32PM -0700, Henry Burns wrote:
> 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.

We already unregister shrinker so there is no upcoming async free call
via shrinker so the only concern is zs_compact API direct call from
the user. Is that what what you desribe from the description?

If so, can't we add a flag to indicate destroy of the pool and
global counter to indicate how many of zs_compact was nested?

So, zs_unregister_migration in zs_destroy_pool can set the flag to
prevent upcoming zs_compact call and wait until the global counter
will be zero. Once it's done, finally flush the work.

My point is it's not a per-class granuarity but global.

Thanks.

> 
> 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.
> 
> Signed-off-by: Henry Burns <henryburns@xxxxxxxxxx>
> ---
>  mm/zsmalloc.c | 68 ++++++++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 65 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index efa660a87787..1f16ed4d6a13 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -53,6 +53,7 @@
>  #include <linux/zpool.h>
>  #include <linux/mount.h>
>  #include <linux/migrate.h>
> +#include <linux/wait.h>
>  #include <linux/pagemap.h>
>  #include <linux/fs.h>
>  
> @@ -206,6 +207,10 @@ struct size_class {
>  	int objs_per_zspage;
>  	/* Number of PAGE_SIZE sized pages to combine to form a 'zspage' */
>  	int pages_per_zspage;
> +#ifdef CONFIG_COMPACTION
> +	/* Number of zspages currently isolated by compaction */
> +	int isolated;
> +#endif
>  
>  	unsigned int index;
>  	struct zs_size_stat stats;
> @@ -267,6 +272,8 @@ struct zs_pool {
>  #ifdef CONFIG_COMPACTION
>  	struct inode *inode;
>  	struct work_struct free_work;
> +	/* A workqueue for when migration races with async_free_zspage() */
> +	struct wait_queue_head migration_wait;
>  #endif
>  };
>  
> @@ -1917,6 +1924,21 @@ static void putback_zspage_deferred(struct zs_pool *pool,
>  
>  }
>  
> +static inline void zs_class_dec_isolated(struct zs_pool *pool,
> +					 struct size_class *class)
> +{
> +	assert_spin_locked(&class->lock);
> +	VM_BUG_ON(class->isolated <= 0);
> +	class->isolated--;
> +	/*
> +	 * There's no possibility of racing, since wait_for_isolated_drain()
> +	 * checks the isolated count under &class->lock after enqueuing
> +	 * on migration_wait.
> +	 */
> +	if (class->isolated == 0 && waitqueue_active(&pool->migration_wait))
> +		wake_up_all(&pool->migration_wait);
> +}
> +
>  static void replace_sub_page(struct size_class *class, struct zspage *zspage,
>  				struct page *newpage, struct page *oldpage)
>  {
> @@ -1986,6 +2008,7 @@ static bool zs_page_isolate(struct page *page, isolate_mode_t mode)
>  	 */
>  	if (!list_empty(&zspage->list) && !is_zspage_isolated(zspage)) {
>  		get_zspage_mapping(zspage, &class_idx, &fullness);
> +		class->isolated++;
>  		remove_zspage(class, zspage, fullness);
>  	}
>  
> @@ -2085,8 +2108,14 @@ static int zs_page_migrate(struct address_space *mapping, struct page *newpage,
>  	 * 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 still hold the class lock while all of this is happening,
> +		 * so we cannot race with zs_destroy_pool()
> +		 */
>  		putback_zspage_deferred(pool, class, zspage);
> +		zs_class_dec_isolated(pool, class);
> +	}
>  
>  	reset_page(page);
>  	put_page(page);
> @@ -2131,9 +2160,11 @@ static void zs_page_putback(struct page *page)
>  
>  	spin_lock(&class->lock);
>  	dec_zspage_isolation(zspage);
> -	if (!is_zspage_isolated(zspage))
> -		putback_zspage_deferred(pool, class, zspage);
>  
> +	if (!is_zspage_isolated(zspage)) {
> +		putback_zspage_deferred(pool, class, zspage);
> +		zs_class_dec_isolated(pool, class);
> +	}
>  	spin_unlock(&class->lock);
>  }
>  
> @@ -2156,8 +2187,36 @@ static int zs_register_migration(struct zs_pool *pool)
>  	return 0;
>  }
>  
> +static bool class_isolated_are_drained(struct size_class *class)
> +{
> +	bool ret;
> +
> +	spin_lock(&class->lock);
> +	ret = class->isolated == 0;
> +	spin_unlock(&class->lock);
> +	return ret;
> +}
> +
> +/* Function for resolving migration */
> +static void wait_for_isolated_drain(struct zs_pool *pool)
> +{
> +	int i;
> +
> +	/*
> +	 * 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 each size_class's isolated
> +	 * count to hit zero.
> +	 */
> +	for (i = 0; i < ZS_SIZE_CLASSES; i++) {
> +		wait_event(pool->migration_wait,
> +			   class_isolated_are_drained(pool->size_class[i]));
> +	}
> +}
> +
>  static void zs_unregister_migration(struct zs_pool *pool)
>  {
> +	wait_for_isolated_drain(pool); /* This can block */
>  	flush_work(&pool->free_work);
>  	iput(pool->inode);
>  }
> @@ -2401,6 +2460,8 @@ struct zs_pool *zs_create_pool(const char *name)
>  	if (!pool->name)
>  		goto err;
>  
> +	init_waitqueue_head(&pool->migration_wait);
> +
>  	if (create_cache(pool))
>  		goto err;
>  
> @@ -2466,6 +2527,7 @@ struct zs_pool *zs_create_pool(const char *name)
>  		class->index = i;
>  		class->pages_per_zspage = pages_per_zspage;
>  		class->objs_per_zspage = objs_per_zspage;
> +		class->isolated = 0;
>  		spin_lock_init(&class->lock);
>  		pool->size_class[i] = class;
>  		for (fullness = ZS_EMPTY; fullness < NR_ZS_FULLNESS;
> -- 
> 2.22.0.770.g0f2c4a37fd-goog
>