Re: [PATCH 1/2] mm/zsmalloc.c: Migration can leave pages in ZS_EMPTY indefinitely

[Minchan Kim <minchan@kernel.org>]

On Thu, Aug 01, 2019 at 06:53:31PM -0700, Henry Burns wrote:
> In zs_page_migrate() we call putback_zspage() after we have finished
> migrating all pages in this zspage. However, the return value is ignored.
> If a zs_free() races in between zs_page_isolate() and zs_page_migrate(),
> freeing the last object in the zspage, putback_zspage() will leave the page
> in ZS_EMPTY for potentially an unbounded amount of time.

Nice catch.

> 
> To fix this, we need to do the same thing as zs_page_putback() does:
> schedule free_work to occur.  To avoid duplicated code, move the
> sequence to a new putback_zspage_deferred() function which both
> zs_page_migrate() and zs_page_putback() call.
> 
> Signed-off-by: Henry Burns <henryburns@google.com>
Cc: <stable@vger.kernel.org>    [4.8+]
Acked-by: Minchan Kim <minchan@kernel.org>

Below a just trivial:

> ---
>  mm/zsmalloc.c | 30 ++++++++++++++++++++----------
>  1 file changed, 20 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
> index 1cda3fe0c2d9..efa660a87787 100644
> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -1901,6 +1901,22 @@ static void dec_zspage_isolation(struct zspage *zspage)
>  	zspage->isolated--;
>  }
>  
> +static void putback_zspage_deferred(struct zs_pool *pool,
> +				    struct size_class *class,
> +				    struct zspage *zspage)
> +{
> +	enum fullness_group fg;
> +
> +	fg = putback_zspage(class, zspage);
> +	/*
> +	 * Due to page_lock, we cannot free zspage immediately
> +	 * so let's defer.
> +	 */

Could you move this comment function's description since it becomes
a function?

Thanks.