Miaohe Lin <linmiaohe@xxxxxxxxxx> writes:
> On 2021/4/19 15:04, Huang, Ying wrote:
>> Miaohe Lin <linmiaohe@xxxxxxxxxx> writes:
>>
>>> On 2021/4/19 10:15, Huang, Ying wrote:
>>>> Miaohe Lin <linmiaohe@xxxxxxxxxx> writes:
>>>>
>>>>> When I was investigating the swap code, I found the below possible race
>>>>> window:
>>>>>
>>>>> CPU 1 CPU 2
>>>>> ----- -----
>>>>> shmem_swapin
>>>>> swap_cluster_readahead
>>>>> if (likely(si->flags & (SWP_BLKDEV | SWP_FS_OPS))) {
>>>>> swapoff
>>>>> si->flags &= ~SWP_VALID;
>>>>> ..
>>>>> synchronize_rcu();
>>>>> ..
>>>>
>>>> You have removed these code in the previous patches of the series. And
>>>> they are not relevant in this patch.
>>>
>>> Yes, I should change these. Thanks.
>>>
>>>>
>>>>> si->swap_file = NULL;
>>>>> struct inode *inode = si->swap_file->f_mapping->host;[oops!]
>>>>>
>>>>> Close this race window by using get/put_swap_device() to guard against
>>>>> concurrent swapoff.
>>>>>
>>>>> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
>>>>
>>>> No. This isn't the commit that introduces the race condition. Please
>>>> recheck your git blame result.
>>>>
>>>
>>> I think this is really hard to find exact commit. I used git blame and found
>>> this race should be existed when this is introduced. Any suggestion ?
>>> Thanks.
>>
>> I think the commit that introduces the race condition is commit
>> 8fd2e0b505d1 ("mm: swap: check if swap backing device is congested or
>> not")
>>
>
> Thanks.
> The commit log only describes one race condition. And for that one, this should be correct
> Fixes tag. But there are still many other race conditions inside swap_cluster_readahead,
> such as swap_readpage() called from swap_cluster_readahead. This tag could not cover the
> all race windows.
No. swap_readpage() in swap_cluster_readahead() is OK. Because
__read_swap_cache_async() is called before that, so the swap entry will
be marked with SWAP_HAS_CACHE, and page will be locked.
Best Regards,
Huang, Ying
>> Best Regards,
>> Huang, Ying
>>
>>>> Best Regards,
>>>> Huang, Ying
>>>>
>>>>> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
>>>>> ---
>>>>> mm/shmem.c | 6 ++++++
>>>>> 1 file changed, 6 insertions(+)
>>>>>
>>>>> diff --git a/mm/shmem.c b/mm/shmem.c
>>>>> index 26c76b13ad23..936ba5595297 100644
>>>>> --- a/mm/shmem.c
>>>>> +++ b/mm/shmem.c
>>>>> @@ -1492,15 +1492,21 @@ static void shmem_pseudo_vma_destroy(struct vm_area_struct *vma)
>>>>> static struct page *shmem_swapin(swp_entry_t swap, gfp_t gfp,
>>>>> struct shmem_inode_info *info, pgoff_t index)
>>>>> {
>>>>> + struct swap_info_struct *si;
>>>>> struct vm_area_struct pvma;
>>>>> struct page *page;
>>>>> struct vm_fault vmf = {
>>>>> .vma = &pvma,
>>>>> };
>>>>>
>>>>> + /* Prevent swapoff from happening to us. */
>>>>> + si = get_swap_device(swap);
>>>>> + if (unlikely(!si))
>>>>> + return NULL;
>>>>> shmem_pseudo_vma_init(&pvma, info, index);
>>>>> page = swap_cluster_readahead(swap, gfp, &vmf);
>>>>> shmem_pseudo_vma_destroy(&pvma);
>>>>> + put_swap_device(si);
>>>>>
>>>>> return page;
>>>>> }
>>>> .
>>>>
>> .
>>
