From: Miaohe Lin <linmiaohe@huawei.com>
To: "Huang, Ying" <ying.huang@intel.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>,
<akpm@linux-foundation.org>, <hannes@cmpxchg.org>,
<mhocko@suse.com>, <iamjoonsoo.kim@lge.com>, <vbabka@suse.cz>,
<alex.shi@linux.alibaba.com>, <willy@infradead.org>,
<minchan@kernel.org>, <richard.weiyang@gmail.com>,
<hughd@google.com>, <linux-kernel@vger.kernel.org>,
<linux-mm@kvack.org>
Subject: Re: [PATCH 2/5] swap: fix do_swap_page() race with swapoff
Date: Mon, 12 Apr 2021 11:24:58 +0800 [thread overview]
Message-ID: <2268e7f9-cfdf-92f3-9a32-04e103e132fc@huawei.com> (raw)
In-Reply-To: <87r1jgwa2w.fsf@yhuang6-desk1.ccr.corp.intel.com>
On 2021/4/12 9:44, Huang, Ying wrote:
> Miaohe Lin <linmiaohe@huawei.com> writes:
>
>> On 2021/4/10 1:17, Tim Chen wrote:
>>>
>>>
>>> On 4/9/21 1:42 AM, Miaohe Lin wrote:
>>>> On 2021/4/9 5:34, Tim Chen wrote:
>>>>>
>>>>>
>>>>> On 4/8/21 6:08 AM, Miaohe Lin wrote:
>>>>>> When I was investigating the swap code, I found the below possible race
>>>>>> window:
>>>>>>
>>>>>> CPU 1 CPU 2
>>>>>> ----- -----
>>>>>> do_swap_page
>>>>>> synchronous swap_readpage
>>>>>> alloc_page_vma
>>>>>> swapoff
>>>>>> release swap_file, bdev, or ...
>>>>>
>>>>
>>>> Many thanks for quick review and reply!
>>>>
>>>>> Perhaps I'm missing something. The release of swap_file, bdev etc
>>>>> happens after we have cleared the SWP_VALID bit in si->flags in destroy_swap_extents
>>>>> if I read the swapoff code correctly.
>>>> Agree. Let's look this more close:
>>>> CPU1 CPU2
>>>> ----- -----
>>>> swap_readpage
>>>> if (data_race(sis->flags & SWP_FS_OPS)) {
>>>> swapoff
>>>> p->swap_file = NULL;
>>>> struct file *swap_file = sis->swap_file;
>>>> struct address_space *mapping = swap_file->f_mapping;[oops!]
>>>> ...
>>>> p->flags = 0;
>>>> ...
>>>>
>>>> Does this make sense for you?
>>>
>>> p->swapfile = NULL happens after the
>>> p->flags &= ~SWP_VALID, synchronize_rcu(), destroy_swap_extents() sequence in swapoff().
>>>
>>> So I don't think the sequence you illustrated on CPU2 is in the right order.
>>> That said, without get_swap_device/put_swap_device in swap_readpage, you could
>>> potentially blow pass synchronize_rcu() on CPU2 and causes a problem. so I think
>>> the problematic race looks something like the following:
>>>
>>>
>>> CPU1 CPU2
>>> ----- -----
>>> swap_readpage
>>> if (data_race(sis->flags & SWP_FS_OPS)) {
>>> swapoff
>>> p->flags = &= ~SWP_VALID;
>>> ..
>>> synchronize_rcu();
>>> ..
>>> p->swap_file = NULL;
>>> struct file *swap_file = sis->swap_file;
>>> struct address_space *mapping = swap_file->f_mapping;[oops!]
>>> ...
>>> ...
>>>
>>
>> Agree. This is also what I meant to illustrate. And you provide a better one. Many thanks!
>
> For the pages that are swapped in through swap cache. That isn't an
> issue. Because the page is locked, the swap entry will be marked with
> SWAP_HAS_CACHE, so swapoff() cannot proceed until the page has been
> unlocked.
>
> So the race is for the fast path as follows,
>
> if (data_race(si->flags & SWP_SYNCHRONOUS_IO) &&
> __swap_count(entry) == 1)
>
> I found it in your original patch description. But please make it more
> explicit to reduce the potential confusing.
Sure. Should I rephrase the commit log to clarify this or add a comment in the code?
Thanks.
>
> Best Regards,
> Huang, Ying
> .
>
next prev parent reply other threads:[~2021-04-12 3:25 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-08 13:08 [PATCH 0/5] close various race windows for swap Miaohe Lin
2021-04-08 13:08 ` [PATCH 1/5] mm/swapfile: add percpu_ref support " Miaohe Lin
2021-04-12 3:30 ` Huang, Ying
2021-04-12 6:59 ` Miaohe Lin
2021-04-12 7:24 ` Huang, Ying
2021-04-13 12:39 ` Miaohe Lin
2021-04-14 1:17 ` Huang, Ying
2021-04-14 1:58 ` Miaohe Lin
2021-04-14 2:06 ` Huang, Ying
2021-04-14 3:44 ` Dennis Zhou
2021-04-14 3:59 ` Huang, Ying
2021-04-14 4:05 ` Dennis Zhou
2021-04-14 5:44 ` Huang, Ying
2021-04-14 14:53 ` Dennis Zhou
2021-04-15 3:16 ` Miaohe Lin
2021-04-15 4:20 ` Dennis Zhou
2021-04-15 9:17 ` Miaohe Lin
2021-04-15 5:24 ` Huang, Ying
2021-04-15 14:31 ` Dennis Zhou
2021-04-16 0:54 ` Huang, Ying
2021-04-16 2:27 ` Miaohe Lin
2021-04-16 6:25 ` Huang, Ying
2021-04-16 8:30 ` Miaohe Lin
2021-04-08 13:08 ` [PATCH 2/5] swap: fix do_swap_page() race with swapoff Miaohe Lin
2021-04-08 21:34 ` Tim Chen
2021-04-09 8:42 ` Miaohe Lin
2021-04-09 17:17 ` Tim Chen
2021-04-10 3:17 ` Miaohe Lin
2021-04-12 1:44 ` Huang, Ying
2021-04-12 3:24 ` Miaohe Lin [this message]
2021-04-13 1:27 ` Huang, Ying
2021-04-13 19:24 ` Tim Chen
2021-04-14 1:04 ` Huang, Ying
2021-04-14 2:20 ` Miaohe Lin
2021-04-14 16:13 ` Tim Chen
2021-04-15 3:19 ` Miaohe Lin
2021-04-14 2:55 ` Miaohe Lin
2021-04-14 3:07 ` Huang, Ying
2021-04-14 3:27 ` Miaohe Lin
2021-04-08 13:08 ` [PATCH 3/5] mm/swap_state: fix get_shadow_from_swap_cache() " Miaohe Lin
2021-04-13 1:33 ` Huang, Ying
2021-04-14 2:42 ` Miaohe Lin
2021-04-08 13:08 ` [PATCH 4/5] mm/swap_state: fix potential faulted in race in swap_ra_info() Miaohe Lin
2021-04-09 8:50 ` Huang, Ying
2021-04-09 9:00 ` Miaohe Lin
2021-04-12 0:55 ` Huang, Ying
2021-04-12 3:17 ` Miaohe Lin
2021-04-08 13:08 ` [PATCH 5/5] mm/swap_state: fix swap_cluster_readahead() race with swapoff Miaohe Lin
2021-04-13 1:36 ` Huang, Ying
2021-04-14 2:43 ` Miaohe Lin
2021-04-08 14:55 ` [PATCH 0/5] close various race windows for swap riteshh
2021-04-09 8:01 ` Miaohe Lin
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