linux-kernel - Re: [PATCH v4 4/4] mm/shmem: fix shmem

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Date:   Sun, 25 Apr 2021 14:27:35 +0800
From:   Miaohe Lin <linmiaohe@...wei.com>
To:     "Huang, Ying" <ying.huang@...el.com>
CC:     <akpm@...ux-foundation.org>, <dennis@...nel.org>,
        <tim.c.chen@...ux.intel.com>, <hughd@...gle.com>,
        <hannes@...xchg.org>, <mhocko@...e.com>, <iamjoonsoo.kim@....com>,
        <alexs@...nel.org>, <willy@...radead.org>, <minchan@...nel.org>,
        <richard.weiyang@...il.com>, <shy828301@...il.com>,
        <david@...hat.com>, <linux-kernel@...r.kernel.org>,
        <linux-mm@...ck.org>
Subject: Re: [PATCH v4 4/4] mm/shmem: fix shmem_swapin() race with swapoff

On 2021/4/25 12:20, Huang, Ying wrote:
> Miaohe Lin <linmiaohe@...wei.com> writes:
> 
>> On 2021/4/25 11:07, Huang, Ying wrote:
>>> I think it's better to put_swap_device() just before returning from the
>>> function.  It's not a big issue to slow down swapoff() a little.  And
>>> this will make the logic easier to be understood.
>>>
>>
>> shmem_swapin_page() already has a methed, i.e. locked page, to prevent races. I was intended
>> to not mix with that. But your suggestion is good as this will make the logic easier to be
>> understood.
>>
>> Just to make sure, is this what you mean? Many thanks!
> 
> Yes.  Just a minor comment.
> 
>>
>> diff --git a/mm/shmem.c b/mm/shmem.c
>> index 26c76b13ad23..737e5b3200c3 100644
>> --- a/mm/shmem.c
>> +++ b/mm/shmem.c
>> @@ -1696,6 +1696,7 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
>>         struct address_space *mapping = inode->i_mapping;
>>         struct shmem_inode_info *info = SHMEM_I(inode);
>>         struct mm_struct *charge_mm = vma ? vma->vm_mm : current->mm;
>> +       struct swap_info_struct *si;
>>         struct page *page;
>>         swp_entry_t swap;
>>         int error;
>> @@ -1704,6 +1705,12 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
>>         swap = radix_to_swp_entry(*pagep);
>>         *pagep = NULL;
>>
>> +       /* Prevent swapoff from happening to us. */
>> +       si = get_swap_device(swap);
>> +       if (unlikely(!si)) {
> 
> I don't think it's necessary to use unlikely() here.
> 

Will do in next version. Thanks!

> Best Regards,
> Huang, Ying
> 
>> +               error = EINVAL;
>> +               goto failed;
>> +       }
>>         /* Look it up and read it in.. */
>>         page = lookup_swap_cache(swap, NULL, 0);
>>         if (!page) {
>> @@ -1765,6 +1772,8 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
>>         swap_free(swap);
>>
>>         *pagep = page;
>> +       if (si)
>> +               put_swap_device(si);
>>         return 0;
>>  failed:
>>         if (!shmem_confirm_swap(mapping, index, swap))
>> @@ -1775,6 +1784,9 @@ static int shmem_swapin_page(struct inode *inode, pgoff_t index,
>>                 put_page(page);
>>         }
>>
>> +       if (si)
>> +               put_swap_device(si);
>> +
>>         return error;
>>  }
>>
>>> Best Regards,
>>> Huang, Ying
>>>
> .
>