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Subject: Re: [PATCH v3 1/3] mm/swapfile: unuse_pte can map random data if swap
 read fails
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References: <20220424091105.48374-1-linmiaohe@huawei.com>
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From:   Miaohe Lin <linmiaohe@huawei.com>
Message-ID: <7b1ee28e-5ed2-6829-4573-035a39791cd7@huawei.com>
Date:   Tue, 10 May 2022 20:46:37 +0800
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On 2022/5/10 14:17, HORIGUCHI NAOYA(堀口 直也) wrote:
> On Sun, Apr 24, 2022 at 05:11:03PM +0800, Miaohe Lin wrote:
>> There is a bug in unuse_pte(): when swap page happens to be unreadable,
>> page filled with random data is mapped into user address space.  In case
>> of error, a special swap entry indicating swap read fails is set to the
>> page table.  So the swapcache page can be freed and the user won't end up
>> with a permanently mounted swap because a sector is bad.  And if the page
>> is accessed later, the user process will be killed so that corrupted data
>> is never consumed.  On the other hand, if the page is never accessed, the
>> user won't even notice it.
>>
>> Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
>> Acked-by: David Hildenbrand <david@redhat.com>
> 
> When I reproduced the issue (generated read error with dm-dust), I saw
> infinite loop in the while loop in shmem_unuse_inode() (and this happens
> even with this patch). I confirmed that shmem_swapin_page() returns -EIO,
> but shmem_unuse_swap_entries() does not return the error to the callers,
> so the while loop in shmem_unuse_inode() seems not break.

In the current implementation, try_to_unuse will keep trying to do shmem_unuse unless -ENOMEM is
returned from shmem_swapin_folio. This could be easily fixed by return -EIO when swapin error
occurs. But the user will end up with a permanently mounted swap just because a sector was bad.
One alternative is inventing a way to proceed the swapoff while preventing user from accessing
the wrong data. But this might complicate the code a lot and I need to learn more about shmem.
Any suggestion will be really grateful!

Thanks! :)

> 
> So maybe you need more code around shmem_unuse_inode() to handle the error?
> 
> Thanks,
> Naoya Horiguchi
>