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Message-ID: <63c96d54-f16b-4a39-a665-227cc194cf96@huawei.com>
Date: Mon, 25 Apr 2022 16:51:02 +0800
From: Miaohe Lin <linmiaohe@...wei.com>
To: David Hildenbrand <david@...hat.com>,
"ying.huang@...el.com" <ying.huang@...el.com>
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>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3 1/3] mm/swapfile: unuse_pte can map random data if swap
read fails
On 2022/4/25 16:01, David Hildenbrand wrote:
> 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 ...
I tend to agree with David. Thanks both!
>
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