Miaohe Lin <linmia...@huawei.com> writes:
> On 2021/4/10 1:17, Tim Chen wrote:
>>
>>
>> On 4/9/21 1:42 AM, Miaohe Lin wrote:
>>> On 2021/4/9 5:34, Tim Chen wrote:
>>>>
>>>>
>>>> On 4/8/21 6:08 AM, Miaohe Lin wrote:
>>>>> When I was investigating the swap code, I found the below possible race
>>>>> window:
>>>>>
>>>>> CPU 1 CPU 2
>>>>> ----- -----
>>>>> do_swap_page
>>>>> synchronous swap_readpage
>>>>> alloc_page_vma
>>>>> swapoff
>>>>> release swap_file, bdev, or ...
>>>>
>>>
>>> Many thanks for quick review and reply!
>>>
>>>> Perhaps I'm missing something. The release of swap_file, bdev etc
>>>> happens after we have cleared the SWP_VALID bit in si->flags in
>>>> destroy_swap_extents
>>>> if I read the swapoff code correctly.
>>> Agree. Let's look this more close:
>>> CPU1 CPU2
>>> ----- -----
>>> swap_readpage
>>> if (data_race(sis->flags & SWP_FS_OPS)) {
>>> swapoff
>>> p->swap_file
>>> = NULL;
>>> struct file *swap_file = sis->swap_file;
>>> struct address_space *mapping = swap_file->f_mapping;[oops!]
>>> ...
>>> p->flags = 0;
>>> ...
>>>
>>> Does this make sense for you?
>>
>> p->swapfile = NULL happens after the
>> p->flags &= ~SWP_VALID, synchronize_rcu(), destroy_swap_extents() sequence
>> in swapoff().
>>
>> So I don't think the sequence you illustrated on CPU2 is in the right order.
>> That said, without get_swap_device/put_swap_device in swap_readpage, you
>> could
>> potentially blow pass synchronize_rcu() on CPU2 and causes a problem. so I
>> think
>> the problematic race looks something like the following:
>>
>>
>> CPU1 CPU2
>> ----- -----
>> swap_readpage
>> if (data_race(sis->flags & SWP_FS_OPS)) {
>> swapoff
>> p->flags = &=
>> ~SWP_VALID;
>> ..
>>
>> synchronize_rcu();
>> ..
>> p->swap_file
>> = NULL;
>> struct file *swap_file = sis->swap_file;
>> struct address_space *mapping = swap_file->f_mapping;[oops!]
>> ...
>> ...
>>
>
> Agree. This is also what I meant to illustrate. And you provide a better one.
> Many thanks!
For the pages that are swapped in through swap cache. That isn't an
issue. Because the page is locked, the swap entry will be marked with
SWAP_HAS_CACHE, so swapoff() cannot proceed until the page has been
unlocked.
So the race is for the fast path as follows,
if (data_race(si->flags & SWP_SYNCHRONOUS_IO) &&
__swap_count(entry) == 1)
I found it in your original patch description. But please make it more
explicit to reduce the potential confusing.
Best Regards,
Huang, Ying
