linux-kernel - Re: [PATCH v2] mm/swapfile: unuse_pte can map random data if swap read fails

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Date:   Tue, 19 Apr 2022 09:39:35 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Miaohe Lin <linmiaohe@...wei.com>,
        Alistair Popple <apopple@...dia.com>
Cc:     akpm@...ux-foundation.org, willy@...radead.org, vbabka@...e.cz,
        dhowells@...hat.com, neilb@...e.de, surenb@...gle.com,
        minchan@...nel.org, peterx@...hat.com, sfr@...b.auug.org.au,
        rcampbell@...dia.com, naoya.horiguchi@....com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] mm/swapfile: unuse_pte can map random data if swap
 read fails

On 19.04.22 09:29, Miaohe Lin wrote:
> On 2022/4/19 11:51, Alistair Popple wrote:
>> Miaohe Lin <linmiaohe@...wei.com> writes:
>>
>>> 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.
>>
>> Hi Miaohe,
>>> It seems we're not actually using the pfn that gets stored in the special swap
>> entry here. Is my understanding correct? If so I think it would be better to use
> 
> Yes, you're right. The pfn is not used now. What we need here is a special swap entry
> to do the right things. I think we can change to store some debugging information instead
> of pfn if needed in the future.
> 
>> the new PTE markers Peter introduced[1] rather than adding another swap entry
>> type.
> 
> IIUC, we should not reuse that swap entry here. From definition:
> 
> PTE markers
> ===========
> ...
> PTE marker is a new type of swap entry that is ony applicable to file
> backed memories like shmem and hugetlbfs.  It's used to persist some
> pte-level information even if the original present ptes in pgtable are
> zapped.
> 
> It's designed for file backed memories while swapin error entry is for anonymous
> memories. And there has some differences in processing. So it's not a good idea
> to reuse pte markers. Or am I miss something?

I tend to agree. As raised in my other reply, maybe we can simply reuse
hwpoison entries and update the documentation of them accordingly.

-- 
Thanks,

David / dhildenb