Re: [PATCH] mm,swap: add document about RCU read lock and swapoff interaction

[Ryan Roberts]

On 07/04/2024 07:54, Huang Ying wrote:
> During reviewing a patch to fix the race condition between
> free_swap_and_cache() and swapoff() [1], it was found that the
> document about how to prevent racing with swapoff isn't clear enough.
> Especially RCU read lock can prevent swapoff from freeing data
> structures.  So, the document is added as comments.
> 
> [1] https://lore.kernel.org/linux-mm/c8fe62d0-78b8-527a-5bef-ee663ccdc37a@xxxxxxxxxx/
> 
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Miaohe Lin <linmiaohe@xxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>

LGTM!

Reviewed-by: Ryan Roberts <ryan.roberts@xxxxxxx>


> ---
>  mm/swapfile.c | 26 +++++++++++++-------------
>  1 file changed, 13 insertions(+), 13 deletions(-)
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 4919423cce76..6925462406fa 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1226,16 +1226,15 @@ 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.
> + * prevent swapoff, such as the folio in swap cache is locked, RCU
> + * reader side is locked, 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.
>   *
> - * Note that when only holding the PTL, swapoff might succeed immediately
> - * after freeing a swap entry. Therefore, immediately after
> - * __swap_entry_free(), the swap info might become stale and should not
> - * be touched without a prior get_swap_device().
> + * RCU reader side lock (including any spinlock) is sufficient to
> + * prevent swapoff, because synchronize_rcu() is called in swapoff()
> + * before freeing data structures.
>   *
>   * Check whether swap entry is valid in the swap device.  If so,
>   * return pointer to swap_info_struct, and keep the swap entry valid
> @@ -2495,10 +2494,11 @@ SYSCALL_DEFINE1(swapoff, const char __user *, specialfile)
>  
>  	/*
>  	 * Wait for swap operations protected by get/put_swap_device()
> -	 * to complete.
> -	 *
> -	 * We need synchronize_rcu() here to protect the accessing to
> -	 * the swap cache data structure.
> +	 * to complete.  Because of synchronize_rcu() here, all swap
> +	 * operations protected by RCU reader side lock (including any
> +	 * spinlock) will be waited too.  This makes it easy to
> +	 * prevent folio_test_swapcache() and the following swap cache
> +	 * operations from racing with swapoff.
>  	 */
>  	percpu_ref_kill(&p->users);
>  	synchronize_rcu();