Re: [PATCH -V3 5/5] swap: comments get_swap_device() with usage rule

[Chris Li]

On Mon, May 29, 2023 at 02:13:55PM +0800, Huang Ying wrote:
> The general rule to use a swap entry is as follows.
> 
> When we get a swap entry, if there aren't some other ways to prevent
> swapoff, such as the folio in swap cache is locked, page table lock is
> held, etc., the swap entry may become invalid because of swapoff.
> Then, we need to enclose all swap related functions with
> get_swap_device() and put_swap_device(), unless the swap functions
> call get/put_swap_device() by themselves.
> 
> Add the rule as comments of get_swap_device().
> 
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
> Reviewed-by: Yosry Ahmed <yosryahmed@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>
> Cc: Chris Li <chrisl@xxxxxxxxxx>
> ---
>  mm/swapfile.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 4dbaea64635d..3d0e932497f0 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1219,6 +1219,13 @@ static unsigned char __swap_entry_free_locked(struct swap_info_struct *p,
>  }
>  
>  /*
> + * When we get a swap entry, if there aren't some other ways to
> + * prevent swapoff, such as the folio in swap cache is locked, page
> + * table lock is held, etc., the swap entry may become invalid because
> + * of swapoff.  Then, we need to enclose all swap related functions
> + * with get_swap_device() and put_swap_device(), unless the swap
> + * functions call get/put_swap_device() by themselves.
> + *
>   * Check whether swap entry is valid in the swap device.  If so,
>   * return pointer to swap_info_struct, and keep the swap entry valid
>   * via preventing the swap device from being swapoff, until
> @@ -1227,9 +1234,8 @@ static unsigned char __swap_entry_free_locked(struct swap_info_struct *p,
>   * Notice that swapoff or swapoff+swapon can still happen before the
>   * percpu_ref_tryget_live() in get_swap_device() or after the
>   * percpu_ref_put() in put_swap_device() if there isn't any other way
> - * to prevent swapoff, such as page lock, page table lock, etc.  The
> - * caller must be prepared for that.  For example, the following
> - * situation is possible.
> + * to prevent swapoff.  The caller must be prepared for that.  For
> + * example, the following situation is possible.
>   *
>   *   CPU1				CPU2
>   *   do_swap_page()
> -- 
> 2.39.2
>

Reviewed-by: Chris Li (Google) <chrisl@xxxxxxxxxx>

Chris