Re: [PATCH] mm/z3fold.c: Fix z3fold_destroy_pool() ordering

[Jonathan Adams <jwadams@google.com>]

On Fri, Jul 26, 2019 at 3:48 PM Henry Burns <henryburns@google.com> wrote:
>
> The constraint from the zpool use of z3fold_destroy_pool() is there are no
> outstanding handles to memory (so no active allocations), but it is possible
> for there to be outstanding work on either of the two wqs in the pool.
>
> If there is work queued on pool->compact_workqueue when it is called,
> z3fold_destroy_pool() will do:
>
>    z3fold_destroy_pool()
>      destroy_workqueue(pool->release_wq)
>      destroy_workqueue(pool->compact_wq)
>        drain_workqueue(pool->compact_wq)
>          do_compact_page(zhdr)
>            kref_put(&zhdr->refcount)
>              __release_z3fold_page(zhdr, ...)
>                queue_work_on(pool->release_wq, &pool->work) *BOOM*
>
> So compact_wq needs to be destroyed before release_wq.
>
> Fixes: 5d03a6613957 ("mm/z3fold.c: use kref to prevent page free/compact race")
>
> Signed-off-by: Henry Burns <henryburns@google.com>

Reviewed-by: Jonathan Adams <jwadams@google.com>

> Cc: <stable@vger.kernel.org>
> ---
>  mm/z3fold.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/mm/z3fold.c b/mm/z3fold.c
> index 1a029a7432ee..43de92f52961 100644
> --- a/mm/z3fold.c
> +++ b/mm/z3fold.c
> @@ -818,8 +818,15 @@ static void z3fold_destroy_pool(struct z3fold_pool *pool)
>  {
>         kmem_cache_destroy(pool->c_handle);
>         z3fold_unregister_migration(pool);
> -       destroy_workqueue(pool->release_wq);
> +
> +       /*
> +        * We need to destroy pool->compact_wq before pool->release_wq,
> +        * as any pending work on pool->compact_wq will call
> +        * queue_work(pool->release_wq, &pool->work).
> +        */
> +
>         destroy_workqueue(pool->compact_wq);
> +       destroy_workqueue(pool->release_wq);
>         kfree(pool);
>  }
>
> --
> 2.22.0.709.g102302147b-goog
>