linux-kernel - Re: [PATCH v3 1/3] mm/swapfile: unuse_pte can map random data if swap read fails

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Date:   Mon, 25 Apr 2022 10:01:06 +0200
From:   David Hildenbrand <david@...hat.com>
To:     "ying.huang@...el.com" <ying.huang@...el.com>,
        Miaohe Lin <linmiaohe@...wei.com>, akpm@...ux-foundation.org
Cc:     willy@...radead.org, vbabka@...e.cz, dhowells@...hat.com,
        neilb@...e.de, apopple@...dia.com, surenb@...gle.com,
        minchan@...nel.org, peterx@...hat.com, sfr@...b.auug.org.au,
        naoya.horiguchi@....com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Tim C Chen <tim.c.chen@...el.com>
Subject: Re: [PATCH v3 1/3] mm/swapfile: unuse_pte can map random data if swap
 read fails

On 25.04.22 09:55, ying.huang@...el.com wrote:
> On Mon, 2022-04-25 at 09:49 +0200, David Hildenbrand wrote:
>> On 25.04.22 09:41, ying.huang@...el.com wrote:
>>> Hi, Miaohe,
>>>
>>> On Sun, 2022-04-24 at 17:11 +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@...wei.com>
>>>> Acked-by: David Hildenbrand <david@...hat.com>
>>>> ---
>>>>  include/linux/swap.h    |  7 ++++++-
>>>>  include/linux/swapops.h | 10 ++++++++++
>>>>  mm/memory.c             |  5 ++++-
>>>>  mm/swapfile.c           | 11 +++++++++++
>>>>  4 files changed, 31 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/include/linux/swap.h b/include/linux/swap.h
>>>> index 5553189d0215..b82c196d8867 100644
>>>> --- a/include/linux/swap.h
>>>> +++ b/include/linux/swap.h
>>>> @@ -55,6 +55,10 @@ static inline int current_is_kswapd(void)
>>>>   * actions on faults.
>>>>   */
>>>>
>>>> +#define SWP_SWAPIN_ERROR_NUM 1
>>>> +#define SWP_SWAPIN_ERROR     (MAX_SWAPFILES + SWP_HWPOISON_NUM + \
>>>> +			     SWP_MIGRATION_NUM + SWP_DEVICE_NUM + \
>>>> +			     SWP_PTE_MARKER_NUM)
>>>>
>>>>
>>>
>>> It appears wasteful to use another swap device number. 
>>
>> Do we really care?
>>
>> We currently use 5 bits for swap types, so we have a total of 32.
>>
>> SWP_HWPOISON_NUM -> 1
>> SWP_MIGRATION_NUM -> 3
>> SWP_PTE_MARKER_NUM -> 1
>> SWP_DEVICE_NUM -> 4
>> SWP_SWAPIN_ERROR_NUM -> 1
>>
>> Which would leave us with 32 - 10 = 22 swap devices. IMHO that's plenty
>> for real life scenarios.
> 
> Creating multiple swap partitions on one disk can improve the
> scalability of swap subsystem, although we usually don't have so many
> disks for swap. 

Exactly, and IMHO if we have 22 or 23 doesn't make a real difference
here ...

-- 
Thanks,

David / dhildenb