Re: [PATCH -V2 4/5] swap: remove get/put_swap_device() in __swap_duplicate()

[Yosry Ahmed]

On Mon, May 22, 2023 at 12:09 AM Huang Ying <ying.huang@xxxxxxxxx> wrote:
>
> __swap_duplicate() is called by
>
> - swap_shmem_alloc(): the page lock of the swap cache is held.
>
> - copy_nonpresent_pte() -> swap_duplicate() and try_to_unmap_one() ->
>   swap_duplicate(): the page table lock is held.
>
> - __read_swap_cache_async() -> swapcache_prepare(): enclosed with
>   get/put_swap_device() in __read_swap_cache_async() already.
>
> So, it's safe to remove get/put_swap_device() in __swap_duplicate().
>
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> Cc: Yang Shi <shy828301@xxxxxxxxx>
> Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
> ---
>  mm/swapfile.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index e9cce775fb25..4dbaea64635d 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -3264,9 +3264,7 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage)

I would add a comment above this function stating that the caller
needs to provide protection against swapoff, and refer to the comment
above get_swap_device().

Otherwise, LGTM with David's comment.

Reviewed-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>

>         unsigned char has_cache;
>         int err;
>
> -       p = get_swap_device(entry);
> -       if (!p)
> -               return -EINVAL;
> +       p = swp_swap_info(entry);
>
>         offset = swp_offset(entry);
>         ci = lock_cluster_or_swap_info(p, offset);
> @@ -3313,7 +3311,6 @@ static int __swap_duplicate(swp_entry_t entry, unsigned char usage)
>
>  unlock_out:
>         unlock_cluster_or_swap_info(p, ci);
> -       put_swap_device(p);
>         return err;
>  }
>
> --
> 2.39.2
>
>