Date:   Wed, 29 Sep 2021 15:50:39 +0800
From:   Rongwei Wang <rongwei.wang@...ux.alibaba.com>
To:     Song Liu <song@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Linux MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        William Kucharski <william.kucharski@...cle.com>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH v2 1/2] mm, thp: check page mapping when truncating page
 cache



On 9/29/21 3:14 PM, Song Liu wrote:
> On Tue, Sep 28, 2021 at 9:20 AM Rongwei Wang
> <rongwei.wang@...ux.alibaba.com> wrote:
>>
>>
>>
>> On 9/28/21 6:24 AM, Song Liu wrote:
>>> On Fri, Sep 24, 2021 at 12:12 AM Rongwei Wang
>>> <rongwei.wang@...ux.alibaba.com> wrote:
>>>>
>>>>
>>>>
>>>> On 9/24/21 10:43 AM, Andrew Morton wrote:
>>>>> On Thu, 23 Sep 2021 01:04:54 +0800 Rongwei Wang <rongwei.wang@...ux.alibaba.com> wrote:
>>>>>
>>>>>>
>>>>>>
>>>>>>> On Sep 22, 2021, at 7:37 PM, Matthew Wilcox <willy@...radead.org> wrote:
>>>>>>>
>>>>>>> On Wed, Sep 22, 2021 at 03:06:44PM +0800, Rongwei Wang wrote:
>>>>>>>> Transparent huge page has supported read-only non-shmem files. The file-
>>>>>>>> backed THP is collapsed by khugepaged and truncated when written (for
>>>>>>>> shared libraries).
>>>>>>>>
>>>>>>>> However, there is race in two possible places.
>>>>>>>>
>>>>>>>> 1) multiple writers truncate the same page cache concurrently;
>>>>>>>> 2) collapse_file rolls back when writer truncates the page cache;
>>>>>>>
>>>>>>> As I've said before, the bug here is that somehow there is a writable fd
>>>>>>> to a file with THPs.  That's what we need to track down and fix.
>>>>>> Hi, Matthew
>>>>>> I am not sure get your means. We know “mm, thp: relax the VM_DENYWRITE constraint on file-backed THPs"
>>>>>> Introduced file-backed THPs for DSO. It is possible {very rarely} for DSO to be opened in writeable way.
>>>>>>
>>>>>> ...
>>>>>>
>>>>>>> https://lore.kernel.org/linux-mm/YUdL3lFLFHzC80Wt@casper.infradead.org/
>>>>>> All in all, what you mean is that we should solve this race at the source?
>>>>>
>>>>> Matthew is being pretty clear here: we shouldn't be permitting
>>>>> userspace to get a writeable fd for a thp-backed file.
>>>>>
>>>>> Why are we permitting the DSO to be opened writeably?  If there's a
>>>>> legitimate case for doing this then presumably "mm, thp: relax the
>>>> There is a use case to stress file-backed THP within attachment.
>>>> I test this case in a system which has enabled CONFIG_READ_ONLY_THP_FOR_FS:
>>>>
>>>> $ gcc -Wall -g -o stress_madvise_dso stress_madvise_dso.c
>>>> $ ulimit -s unlimited
>>>> $ ./stress_madvise_dso 10000 <libtest.so>
>>>>
>>>> the meaning of above parameters:
>>>> 10000: the max test time;
>>>> <libtest.so>: the DSO that will been mapped into file-backed THP by
>>>> madvise. It recommended that the text segment of DSO to be tested is
>>>> greater than 2M.
>>>>
>>>> The crash will been triggered at once in the latest kernel. And this
>>>> case also can used to trigger the bug that mentioned in our another patch.
>>>
>>> Hmm.. I am not able to use the repro program to crash the system. Not
>>> sure what I did wrong.
>>>
>> Hi
>> I have tried to check my test case again. Can you make sure the DSO that
>> you test have THP mapping?
>>
>> If you are willing to try again, I can send my libtest.c which is used
>> to test by myself (actually, it shouldn't be target DSO problem).
>>
>> Thanks very much!
>>> OTOH, does it make sense to block writes within khugepaged, like:
>>>
>>> diff --git i/mm/khugepaged.c w/mm/khugepaged.c
>>> index 045cc579f724e..ad7c41ec15027 100644
>>> --- i/mm/khugepaged.c
>>> +++ w/mm/khugepaged.c
>>> @@ -51,6 +51,7 @@ enum scan_result {
>>>           SCAN_CGROUP_CHARGE_FAIL,
>>>           SCAN_TRUNCATED,
>>>           SCAN_PAGE_HAS_PRIVATE,
>>> +       SCAN_BUSY_WRITE,
>>>    };
>>>
>>>    #define CREATE_TRACE_POINTS
>>> @@ -1652,6 +1653,11 @@ static void collapse_file(struct mm_struct *mm,
>>>           /* Only allocate from the target node */
>>>           gfp = alloc_hugepage_khugepaged_gfpmask() | __GFP_THISNODE;
>>>
>>> +       if (deny_write_access(file)) {
>>> +               result = SCAN_BUSY_WRITE;
>>> +               return;
>>> +       }
>>> +
>> This can indeed avoid some possible races from source.
>>
>> But, I am thinking about whether this will lead to DDoS attack?
>> I remember the reason of DSO has ignored MAP_DENYWRITE in kernel
>> is that DDoS attack. In addition, 'deny_write_access' will change
>> the behavior, such as user will get 'Text file busy' during
>> collapse_file. I am not sure whether the behavior changing is acceptable
>> in user space.
>>
>> If it is acceptable, I am very willing to fix the races like your way.
> 
> I guess we should not let the write get ETXTBUSY for khugepaged work.
> 
> I am getting some segfault on stress_madvise_dso. And it doesn't really
> generate the bug stack in my vm (qemu-system-x86_64). Is there an newer
Hi, I can sure I am not update the stress_madvise_dso.c.

My test environment is vm (qemu-system-aarch64, 32 cores). And I can 
think of the following possibilities:

(1) in thread_read()

printf("read %s\n", dso_path);
d = open(dso_path, O_RDONLY);
/* The start addr must be alignment with 2M */
void *p = mmap((void *)0x40000dc00000UL, 0x800000, PROT_READ | 
PROT_EXEC,MAP_PRIVATE, fd, 0);
if (p == MAP_FAILED) {
	perror("mmap");
	goto out;
}

0x40000dc00000 is random setting by myself. I am not sure this address 
is available in your vm.

(2) in thread_write()
int fd = open(dso_path, O_RDWR);
p = mmap(NULL, 0x800000, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
if (p == MAP_FAILED) {
	perror("mmap");
         goto out; /* fail */
}

because of I am sure the DSO is bigger than 0x800000, so directly map 
the DSO using 0x800000. Maybe I had use '-z max-page-size=0x200000' to 
compile the DSO? likes:
$ gcc -z max-page-size=0x200000 -o libtest.so -shared libtest.o

If you don't mind, you can send the segment fault log to me. And I will
find x86 environment to test.

Thanks!
> version of it?
> 
> Thanks,
> Song
> 

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