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 18:08:38 +1000
From:   Alistair Popple <apopple@...dia.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Miaohe Lin <linmiaohe@...wei.com>, 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

David Hildenbrand <david@...hat.com> writes:

> 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.

Unless I've missed something I don't think PTE markers should be restricted
solely to file backed memory. It's true that the only user of them at the moment
is UFFD-WP for file backed memory, but PTE markers are just a special swap entry
same as what is added here.

That said I don't think there has been any attempt to make PTE markers work for
anything other than UFFD-WP because it was unclear if there ever would be
another user.

But I agree re-using hwpoison entries is probably a better fit if possible.

- Alistair