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SubjectRe: [Question] Is there race between swapoff and swapout
Date
Chen Wandun <chenwandun@huawei.com> writes:

> I analysied the code about swapoff and swapout, and I suspected there may be a race.
> The kernel version is 4.14 stable.
>
> CPU0 CPU1
> swapoff swap out
> add_to_swap
> get_swap_page
> ...... get_swap_pages
> spin_lock(&swap_avail_lock)
> get swap_info_struct
> spin_unlock(&swap_avail_lock)
> spin_lock(&swap_avail_lock)
> __def_from_avail_list(swap_info_struct)
> spin_unlock(&swap_avail_lock) ......
> try_to_unuse // unuse all slot
> /* get a free slot from swap_info_struct,
> * and write data to slot later
> */
> scan_swap_map_slots
> free swap_info_struct
> .......
>
>
> If CPU1 get the swap_info_struct first, then CPU0 delete it from list and
> unuse all slot in swap_info_struct, before CPU0 free swap_info_struct CPU1
> call scan_swap_map_slots to alloc a free slot.
>
> I am not sure the analysis above is correct,
> Please let me know if there is any mistake

SWP_WRITEOK will be cleared during swapoff, and it is checked during
swap slots allocation.

Best Regards,
Huang, Ying

> Thanks
> ChenWandun

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