Re: [PATCH -V2 5/5] swap: comments get_swap_device() with usage rule

[David Hildenbrand <david@redhat.com>]

On 22.05.23 09:09, Huang Ying wrote:
> The general rule to use a swap entry is as follows.
> 
> When we get a swap entry, if there isn't some other way to prevent
> swapoff, such as page lock for swap cache, page table lock, 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@intel.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Minchan Kim <minchan@kernel.org>
> Cc: Tim Chen <tim.c.chen@linux.intel.com>
> Cc: Yang Shi <shy828301@gmail.com>
> Cc: Yu Zhao <yuzhao@google.com>
> ---
>   mm/swapfile.c | 12 +++++++++---
>   1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 4dbaea64635d..0c1cb935b2eb 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 isn't some other way to prevent
> + * swapoff, such as page lock for swap cache, page table lock, etc.,

Again "page lock for swap cache" might be imprecise.

> + * 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()

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb