Re: [PATCH v5 4/4] mm/shmem: fix shmem_swapin() race with swapoff

[Miaohe Lin]

On 2021/4/26 14:53, Yu Zhao wrote:
> On Sun, Apr 25, 2021 at 3:54 AM Miaohe Lin <linmiaohe@xxxxxxxxxx> wrote:
>>
>> 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->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: 8fd2e0b505d1 ("mm: swap: check if swap backing device is congested or not")
>> Signed-off-by: Miaohe Lin <linmiaohe@xxxxxxxxxx>
>> ---
>>  mm/shmem.c | 12 ++++++++++++
>>  1 file changed, 12 insertions(+)
>>
>> diff --git a/mm/shmem.c b/mm/shmem.c
>> index 26c76b13ad23..2dafd65b0b42 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -1696,6 +1696,7 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
>>         struct address_space *mapping = inode->i_mapping;
>>         struct shmem_inode_info *info = SHMEM_I(inode);
>>         struct mm_struct *charge_mm = vma ? vma->vm_mm : current->mm;
>> +       struct swap_info_struct *si;
>>         struct page *page;
>>         swp_entry_t swap;
>>         int error;
>> @@ -1704,6 +1705,12 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
>>         swap = radix_to_swp_entry(*pagep);
>>         *pagep = NULL;
>>
>> +       /* Prevent swapoff from happening to us. */
>> +       si = get_swap_device(swap);
>> +       if (!si) {
>> +               error = EINVAL;
>> +               goto failed;
>> +       }
> 
> page is uninitialized?
> 

Sorry, my overlook! Compiler should have complained about it but there is none...
Many thanks for pointing this out! Will fix it in next version.

>>         /* Look it up and read it in.. */
>>         page = lookup_swap_cache(swap, NULL, 0);
>>         if (!page) {
>> @@ -1765,6 +1772,8 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
>>         swap_free(swap);
>>
>>         *pagep = page;
>> +       if (si)
>> +               put_swap_device(si);
>>         return 0;
>>  failed:
>>         if (!shmem_confirm_swap(mapping, index, swap))
>> @@ -1775,6 +1784,9 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
>>                 put_page(page);
>>         }
>>
>> +       if (si)
>> +               put_swap_device(si);
>> +
>>         return error;
>>  }
>>
>> --
>> 2.23.0
>>
>>
> .
>