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Message-ID: <87sgnih6gz.fsf@yhuang-dev.intel.com>
Date: Thu, 24 Oct 2019 16:51:08 +0800
From: "Huang\, Ying" <ying.huang@...el.com>
To: Chen Wandun <chenwandun@...wei.com>
Cc: <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>
Subject: Re: [Question] Is there race between swapoff and swapout
Chen Wandun <chenwandun@...wei.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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