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
>>
>>
> .
>
