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.
> 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;
>>>> }
>>> .
>>>
> .
>
