From: "Huang, Ying" <ying.huang@intel.com>
To: Miaohe Lin <linmiaohe@huawei.com>
Cc: Linux-MM <linux-mm@kvack.org>,
linux-kernel <linux-kernel@vger.kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
Matthew Wilcox <willy@infradead.org>,
Yu Zhao <yuzhao@google.com>,
"Shakeel Butt" <shakeelb@google.com>,
Alex Shi <alex.shi@linux.alibaba.com>,
"Minchan Kim" <minchan@kernel.org>
Subject: Re: [Question] Is there a race window between swapoff vs synchronous swap_readpage
Date: Tue, 30 Mar 2021 11:44:35 +0800 [thread overview]
Message-ID: <87h7kt9ufw.fsf@yhuang6-desk1.ccr.corp.intel.com> (raw)
In-Reply-To: <0cb765aa-1783-cd62-c4a4-b3fbc620532d@huawei.com> (Miaohe Lin's message of "Tue, 30 Mar 2021 11:15:52 +0800")
Miaohe Lin <linmiaohe@huawei.com> writes:
> On 2021/3/30 9:57, Huang, Ying wrote:
>> Hi, Miaohe,
>>
>> Miaohe Lin <linmiaohe@huawei.com> writes:
>>
>>> Hi all,
>>> I am investigating the swap code, and I found the below possible race window:
>>>
>>> CPU 1 CPU 2
>>> ----- -----
>>> do_swap_page
>>> skip swapcache case (synchronous swap_readpage)
>>> alloc_page_vma
>>> swapoff
>>> release swap_file, bdev, or ...
>>> swap_readpage
>>> check sis->flags is ok
>>> access swap_file, bdev or ...[oops!]
>>> si->flags = 0
>>>
>>> The swapcache case is ok because swapoff will wait on the page_lock of swapcache page.
>>> Is this will really happen or Am I miss something ?
>>> Any reply would be really grateful. Thanks! :)
>>
>> This appears possible. Even for swapcache case, we can't guarantee the
>
> Many thanks for reply!
>
>> swap entry gotten from the page table is always valid too. The
>
> The page table may change at any time. And we may thus do some useless work.
> But the pte_same() check could handle these races correctly if these do not
> result in oops.
>
>> underlying swap device can be swapped off at the same time. So we use
>> get/put_swap_device() for that. Maybe we need similar stuff here.
>
> Using get/put_swap_device() to guard against swapoff for swap_readpage() sounds
> really bad as swap_readpage() may take really long time. Also such race may not be
> really hurtful because swapoff is usually done when system shutdown only.
> I can not figure some simple and stable stuff out to fix this. Any suggestions or
> could anyone help get rid of such race?
Some reference counting on the swap device can prevent swap device from
swapping-off. To reduce the performance overhead on the hot-path as
much as possible, it appears we can use the percpu_ref.
Best Regards,
Huang, Ying
next prev parent reply other threads:[~2021-03-30 3:44 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-29 13:18 [Question] Is there a race window between swapoff vs synchronous swap_readpage Miaohe Lin
2021-03-30 1:57 ` Huang, Ying
2021-03-30 3:15 ` Miaohe Lin
2021-03-30 3:44 ` Huang, Ying [this message]
2021-03-30 5:47 ` Yu Zhao
2021-03-30 6:57 ` Huang, Ying
2021-03-30 7:27 ` Yu Zhao
2021-04-12 3:12 ` Miaohe Lin
2021-03-30 11:21 ` Miaohe Lin
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