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

[Huang, Ying]

David Hildenbrand <david@xxxxxxxxxx> writes:

> 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@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 | 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.

Sure.  Will revise this.

>> + * 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@xxxxxxxxxx>

Thanks!

Best Regards,
Huang, Ying