Re: [PATCH v1] mm: swap: Fix race between free_swap_and_cache() and swapoff()

[Ryan Roberts <ryan.roberts@arm.com>]

On 05/03/2024 22:05, David Hildenbrand wrote:
> On 05.03.24 17:33, Ryan Roberts wrote:
>> On 05/03/2024 15:50, David Hildenbrand wrote:
>>> On 05.03.24 16:13, Ryan Roberts wrote:
>>>> There was previously a theoretical window where swapoff() could run and
>>>> teardown a swap_info_struct while a call to free_swap_and_cache() was
>>>> running in another thread. This could cause, amongst other bad
>>>> possibilities, swap_page_trans_huge_swapped() (called by
>>>> free_swap_and_cache()) to access the freed memory for swap_map.
>>>>
>>>> This is a theoretical problem and I haven't been able to provoke it from
>>>> a test case. But there has been agreement based on code review that this
>>>> is possible (see link below).
>>>>
>>>> Fix it by using get_swap_device()/put_swap_device(), which will stall
>>>> swapoff(). There was an extra check in _swap_info_get() to confirm that
>>>> the swap entry was valid. This wasn't present in get_swap_device() so
>>>> I've added it. I couldn't find any existing get_swap_device() call sites
>>>> where this extra check would cause any false alarms.
>>>>
>>>> Details of how to provoke one possible issue (thanks to David Hilenbrand
>>>> for deriving this):
>>>
>>> Almost
>>>
>>> "s/Hilenbrand/Hildenbrand/" :)
>>
>> Ahh sorry... I even explicitly checked it against your name on emails... fat
>> fingers...
> 
> No need to be sorry. Even your average German person would get it wrong,
> because there are other (more common) variants :)
> 
> [...]
> 
>>>>
>>>
>>> LGTM
>>>
>>> Are you planning on sending a doc extension for get_swap_device()?
>>
>> I saw your comment:
>>
>> We should likely update the documentation of get_swap_device(), that after
>> decrementing the refcount, the SI might become stale and should not be touched
>> without a prior get_swap_device().
>>
>> But when I went to make the changes, I saw the documentation already said:
>>
>> ...we need to enclose all swap related functions with get_swap_device() and
>> put_swap_device()... Notice that swapoff ... can still happen before the
>> percpu_ref_tryget_live() in get_swap_device() or after the percpu_ref_put() in
>> put_swap_device()... The caller must be prepared for that.
>>
>> I thought that already covered it? I'm sure as usual, I've misunderstood your
>> point. Happy to respin if you have something in mind?
> 
> No need to respin, we could clarify on top, if we decide it makes sense.
> 
> I was thinking about something like this, making it clearer that the PTL
> discussion above does not express the corner case we discovered:
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 2b3a2d85e350b..646a436581eee 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -1232,6 +1232,11 @@ static unsigned char __swap_entry_free_locked(struct
> swap_info_struct *p,
>   * 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().
> + *

Are yes, this is useful. I'm going to have to respin anyway, so will include it
in the next version. Thanks!


>   * 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
> 
>