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Subject: Re: [PATCH 3/5] mm/swap_state: fix get_shadow_from_swap_cache() race
 with swapoff
To:     "Huang, Ying" <ying.huang@intel.com>
CC:     <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>, <tim.c.chen@linux.intel.com>,
        <linux-kernel@vger.kernel.org>, <linux-mm@kvack.org>
References: <20210408130820.48233-1-linmiaohe@huawei.com>
 <20210408130820.48233-4-linmiaohe@huawei.com>
 <87im4rufwq.fsf@yhuang6-desk1.ccr.corp.intel.com>
From:   Miaohe Lin <linmiaohe@huawei.com>
Message-ID: <4ebc0a78-6fde-9d43-7562-8ef7679b5460@huawei.com>
Date:   Wed, 14 Apr 2021 10:42:38 +0800
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On 2021/4/13 9:33, Huang, Ying wrote:
> Miaohe Lin <linmiaohe@huawei.com> writes:
> 
>> The function get_shadow_from_swap_cache() can race with swapoff, though
>> it's only called by do_swap_page() now.
>>
>> Fixes: aae466b0052e ("mm/swap: implement workingset detection for anonymous LRU")
>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
> 
> This is unnecessary.  The only caller has guaranteed the swap device
> from swapoff

Ok. This patch is just in case get_shadow_from_swap_cache() would be
called from elsewhere someday.

It's unnecessary and can be dropped now

.
> 
> Best Regards,
> Huang, Ying
> 
>> ---
>>  mm/swap_state.c | 9 ++++++---
>>  1 file changed, 6 insertions(+), 3 deletions(-)
>>
>> diff --git a/mm/swap_state.c b/mm/swap_state.c
>> index 272ea2108c9d..709c260d644a 100644
>> --- a/mm/swap_state.c
>> +++ b/mm/swap_state.c
>> @@ -83,11 +83,14 @@ void show_swap_cache_info(void)
>>  
>>  void *get_shadow_from_swap_cache(swp_entry_t entry)
>>  {
>> -	struct address_space *address_space = swap_address_space(entry);
>> -	pgoff_t idx = swp_offset(entry);
>> +	struct swap_info_struct *si;
>>  	struct page *page;
>>  
>> -	page = xa_load(&address_space->i_pages, idx);
>> +	si = get_swap_device(entry);
>> +	if (!si)
>> +		return NULL;
>> +	page = xa_load(&swap_address_space(entry)->i_pages, swp_offset(entry));
>> +	put_swap_device(si);
>>  	if (xa_is_value(page))
>>  		return page;
>>  	return NULL;
> 
> .
>