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

[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.
>
> Calling z3fold_deregister_migration() before the workqueues are drained
> means that there can be allocated pages referencing a freed inode,
> causing any thread in compaction to be able to trip over the bad
> pointer in PageMovable().
>
> Fixes: 1f862989b04a ("mm/z3fold.c: support page migration")
>
> Signed-off-by: Henry Burns <henryburns@google.com>

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

> Cc: <stable@vger.kernel.org>
> ---
>  mm/z3fold.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/mm/z3fold.c b/mm/z3fold.c
> index 43de92f52961..ed19d98c9dcd 100644
> --- a/mm/z3fold.c
> +++ b/mm/z3fold.c
> @@ -817,16 +817,19 @@ static struct z3fold_pool *z3fold_create_pool(const char *name, gfp_t gfp,
>  static void z3fold_destroy_pool(struct z3fold_pool *pool)
>  {
>         kmem_cache_destroy(pool->c_handle);
> -       z3fold_unregister_migration(pool);
>
>         /*
>          * 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).
> +        *
> +        * There are still outstanding pages until both workqueues are drained,
> +        * so we cannot unregister migration until then.
>          */
>
>         destroy_workqueue(pool->compact_wq);
>         destroy_workqueue(pool->release_wq);
> +       z3fold_unregister_migration(pool);
>         kfree(pool);
>  }
>
> --
> 2.22.0.709.g102302147b-goog
>