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SubjectRe: [PATCH v2] filemap: avoid unnecessary major faults in filemap_fault()
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On 2024/2/26 16:20, David Hildenbrand wrote:

> On 26.02.24 08:52, Huang, Ying wrote:
>> "zhangpeng (AS)" <zhangpeng362@huawei.com> writes:
>>
>>> On 2024/2/26 14:04, Huang, Ying wrote:
>>>
>>>> "zhangpeng (AS)" <zhangpeng362@huawei.com> writes:
>>>>
>>>>> On 2024/2/7 10:21, Huang, Ying wrote:
>>>>>
>>>>>> Peng Zhang <zhangpeng362@huawei.com> writes:
>>>>>>> From: ZhangPeng <zhangpeng362@huawei.com>
>>>>>>>
>>>>>>> The major fault occurred when using mlockall(MCL_CURRENT |
>>>>>>> MCL_FUTURE)
>>>>>>> in application, which leading to an unexpected performance
>>>>>>> issue[1].
>>>>>>>
>>>>>>> This caused by temporarily cleared PTE during a
>>>>>>> read+clear/modify/write
>>>>>>> update of the PTE, eg, do_numa_page()/change_pte_range().
>>>>>>>
>>>>>>> For the data segment of the user-mode program, the global
>>>>>>> variable area
>>>>>>> is a private mapping. After the pagecache is loaded, the private
>>>>>>> anonymous
>>>>>>> page is generated after the COW is triggered. Mlockall can lock
>>>>>>> COW pages
>>>>>>> (anonymous pages), but the original file pages cannot be locked
>>>>>>> and may
>>>>>>> be reclaimed. If the global variable (private anon page) is
>>>>>>> accessed when
>>>>>>> vmf->pte is zeroed in numa fault, a file page fault will be
>>>>>>> triggered.
>>>>>>>
>>>>>>> At this time, the original private file page may have been
>>>>>>> reclaimed.
>>>>>>> If the page cache is not available at this time, a major fault
>>>>>>> will be
>>>>>>> triggered and the file will be read, causing additional overhead.
>>>>>>>
>>>>>>> Fix this by rechecking the PTE without acquiring PTL in
>>>>>>> filemap_fault()
>>>>>>> before triggering a major fault.
>>>>>>>
>>>>>>> Testing file anonymous page read and write page fault
>>>>>>> performance in ext4
>>>>>>> and ramdisk using will-it-scale[2] on a x86 physical machine.
>>>>>>> The data
>>>>>>> is the average change compared with the mainline after the patch is
>>>>>>> applied. The test results are within the range of fluctuation,
>>>>>>> and there
>>>>>>> is no obvious difference. The test results are as follows:
>>>>>> You still claim that there's no difference in the test results. 
>>>>>> If so,
>>>>>> why do you implement the patch?  IMHO, you need to prove your
>>>>>> patch can
>>>>>> improve the performance in some cases.
>>>>> I'm sorry that maybe I didn't express myself clearly.
>>>>>
>>>>> The purpose of this patch is to fix the issue that major fault may
>>>>> still be triggered
>>>>> with mlockall(), thereby improving a little performance. This
>>>>> patch is more of a bugfix
>>>>> than a performance improvement patch.
>>>>>
>>>>> This issue affects our traffic analysis service. The inbound
>>>>> traffic is heavy. If a major
>>>>> fault occurs, the I/O schedule is triggered and the original I/O
>>>>> is suspended. Generally,
>>>>> the I/O schedule is 0.7 ms. If other applications are operating
>>>>> disks, the system needs
>>>>> to wait for more than 10 ms. However, the inbound traffic is heavy
>>>>> and the NIC buffer is
>>>>> small. As a result, packet loss occurs. The traffic analysis
>>>>> service can't tolerate packet
>>>>> loss.
>>>>>
>>>>> To prevent packet loss, we use the mlockall() function to prevent
>>>>> I/O. It is unreasonable
>>>>> that major faults will still be triggered after mlockall() is used.
>>>>>
>>>>> In our service test environment, the baseline is 7 major faults/12
>>>>> hours. After applied the
>>>>> unlock patch, the probability of triggering the major fault is 1
>>>>> major faults/12 hours. After
>>>>> applied the lock patch, no major fault will be triggered. So only
>>>>> the locked patch can actually
>>>>> solve our problem.
>>>> This is the data I asked for.
>>>>
>>>> But, you said that this is a feature bug fix instead of performance
>>>> improvement.  So, I checked the mlock(2), and found the following
>>>> words,
>>>>
>>>> "
>>>>          mlockall() locks all pages mapped into the address space
>>>> of the calling
>>>>          process.  This includes the pages of the code, data, and
>>>> stack segment,
>>>>          as well as shared libraries, user space kernel data,
>>>> shared memory, and
>>>>          memory-mapped files.  All mapped pages are guaranteed to
>>>> be resident in
>>>>          RAM when the call returns successfully; the  pages are 
>>>> guaranteed  to
>>>>          stay in RAM until later unlocked.
>>>> "
>>>>
>>>> In theory, the locked page are in RAM.  So, IIUC, we don't violate the
>>>> ABI.  But, in effect, we does do that.
>>>
>>> "mlockall() locks all pages mapped into the address space of the
>>> calling process."
>>> For a private mapping, mlockall() can lock COW pages (anonymous
>>> pages), but the
>>> original file pages can't be locked. Maybe, we violate the ABI here.
>>
>> If so, please make it explicit and loudly.
>>
>>> This is also
>>> the cause of this issue. The patch fix the impact of this issue:
>>> prevent major
>>> faults, reduce IO, and fix the service packet loss issue.
>>>
>>> Preventing major faults, and thus reducing IO, could be an important
>>> reason to use
>>> mlockall(). Could we fix this with the locked patch? Or is there
>>> another way?
>>
>> Unfortunately, locked patch cause performance regressions for more
>> common cases.  Is it possible for us to change ptep_modify_prot_start()
>> to use some magic PTE value instead of 0?  That may be possible.  But,
>> that isn't enough, you need to change all ptep_get_and_clear() users.
>
> Trigger (false) major faults for mlocked memory is suboptimal.
>
> Having such pages temporarily not mapped (e.g., page migration) is
> acceptable (pages are in RAM but are getting moved). We handle that
> using nonswap migration entries.
>
> Let me understand the issue first:
>
> 1) MAP_PRIVATE file mapping that is mlocked.
>
> 2) We caused COW, so we now have an anonymous page mapped. That anon
>    page is mlocked.
>
> 3) Change of protection (under PT lock) will temporarily clear the PTE
>
> 4) Page fault will trigger and find the PTE still cleared (without PT
>    lock)
>
> 5) We don't realize that there is a page mapped and, therefore, trigger
>    a major fault.
>
> Using the PT lock would fix it properly. Doing it as in this patch can
> only be considered an optimization, not a proper fix.
>
> Using a magic PTE to work around "just use the PT lock like everyone
> else" feels a bit odd. The patch states "We don't hold PTL here as
> acquiring PTL hurts performance" -- do we have any numbers on that?
>
Testing file anonymous page read and write page fault performance in ext4
, tmpfs and ramdisk using will-it-scale[2] on a x86 physical machine. The
data is the average change compared with the mainline after the patch is
applied.

with the locked patch:
processes processes_idle threads threads_idle
ext4 private file write: -0.51% 0.08% -0.03% -0.04%
ext4 shared file write: 0.135% -0.531% 2.883% -0.772%
ramdisk private file write: -0.48% 0.23% -1.08% 0.27%
ramdisk private file read: 0.07% -6.90% -5.85% -0.70%
tmpfs private file write: -0.344% -0.110% 0.200% 0.145%
tmpfs shared file write: 0.958% 0.101% 2.781% -0.337%
tmpfs private file read: -0.16% 0.00% -0.12% 0.41%

with the no locked patch:
processes processes_idle threads threads_idle
ext4 private file write: -1.14% -0.08% -1.87% 0.13%
ext4 private file read: 0.03% -0.65% -0.51% -0.08%
ramdisk private file write: -1.21% -0.21% -1.12% 0.11%
ramdisk private file read: 0.00% -0.68% -0.33% -0.02%

I could also run other tests if needed.

> We could special-case that for MLOCK'ed VMAs with MCL_FUTURE, meaning,
> take the PTL to double-check only in such VMAs.
>
Agreed. I think this solution is great. Thanks for your suggestion!

--
Best Regards,
Peng


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