Date: Mon, 26 Feb 2024 16:19:46 +0800
From: "zhangpeng (AS)" <zhangpeng362@...wei.com>
To: "Huang, Ying" <ying.huang@...el.com>
CC: <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
	<akpm@...ux-foundation.org>, <willy@...radead.org>, <fengwei.yin@...el.com>,
	<aneesh.kumar@...ux.ibm.com>, <shy828301@...il.com>, <hughd@...gle.com>,
	<david@...hat.com>, <wangkefeng.wang@...wei.com>, Nanyong Sun
	<sunnanyong@...wei.com>
Subject: Re: [PATCH v2] filemap: avoid unnecessary major faults in
 filemap_fault()

On 2024/2/26 15:52, Huang, Ying wrote:

> "zhangpeng (AS)" <zhangpeng362@...wei.com> writes:
>
>> On 2024/2/26 14:04, Huang, Ying wrote:
>>
>>> "zhangpeng (AS)" <zhangpeng362@...wei.com> writes:
>>>
>>>> On 2024/2/7 10:21, Huang, Ying wrote:
>>>>
>>>>> Peng Zhang <zhangpeng362@...wei.com> writes:
>>>>>> From: ZhangPeng <zhangpeng362@...wei.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.

Thank you very much for your suggestion!
I'll try it this way.

> --
> Best Regards,
> Huang, Ying
>
>>> But, from git history, we have cleared the PTE during modification from
>>> 2.6.12-rc2 at least.  I guess that because Linux isn't a hard real time
>>> OS, users don't expect that too.
>>>
>>> --
>>> Best Regards,
>>> Huang, Ying
>>>
>>>> The test data provided is intended to prove that the patch does not have a major impact
>>>> on the performance of the page fault itself.
>>>>
>>>>>> 	                 processes processes_idle threads threads_idle
>>>>>> ext4    private file write: -1.14%  -0.08%         -1.87%   0.13%
>>>>>> ext4    shared  file write:  0.14%  -0.53%          2.88%  -0.77%
>>>>>> ext4    private file  read:  0.03%  -0.65%         -0.51%  -0.08%
>>>>>> tmpfs   private file write: -0.34%  -0.11%          0.20%   0.15%
>>>>>> tmpfs   shared  file write:  0.96%   0.10%          2.78%  -0.34%
>>>>>> ramdisk private file write: -1.21%  -0.21%         -1.12%   0.11%
>>>>>> ramdisk private file  read:  0.00%  -0.68%         -0.33%  -0.02%
>>>>>>
>>>>>> [1] https://lore.kernel.org/linux-mm/9e62fd9a-bee0-52bf-50a7-498fa17434ee@huawei.com/
>>>>>> [2] https://github.com/antonblanchard/will-it-scale/
>>>>>>
>>>>>> Suggested-by: "Huang, Ying" <ying.huang@...el.com>
>>>>>> Suggested-by: Yin Fengwei <fengwei.yin@...el.com>
>>>>>> Signed-off-by: ZhangPeng <zhangpeng362@...wei.com>
>>>>>> Signed-off-by: Kefeng Wang <wangkefeng.wang@...wei.com>
>>>>>> ---
>>>>>> v1->v2:
>>>>>> - Add more test results per Huang, Ying
>>>>>> - Add more comments before check PTE per Huang, Ying, David Hildenbrand
>>>>>>      and Yin Fengwei
>>>>>> - Change pte_offset_map_nolock to pte_offset_map as the ptl lock won't
>>>>>>      be used
>>>>>>
>>>>>> RFC->v1:
>>>>>> - Add error handling when ptep == NULL per Huang, Ying and Matthew
>>>>>>      Wilcox
>>>>>> - Check the PTE without acquiring PTL in filemap_fault(), suggested by
>>>>>>      Huang, Ying and Yin Fengwei
>>>>>> - Add pmd_none() check before PTE map
>>>>>> - Update commit message and add performance test information
>>>>>>
>>>>>>     mm/filemap.c | 34 ++++++++++++++++++++++++++++++++++
>>>>>>     1 file changed, 34 insertions(+)
>>>>>>
>>>>>> diff --git a/mm/filemap.c b/mm/filemap.c
>>>>>> index 142864338ca4..a2c1a98bc771 100644
>>>>>> --- a/mm/filemap.c
>>>>>> +++ b/mm/filemap.c
>>>>>> @@ -3238,6 +3238,40 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
>>>>>>     			mapping_locked = true;
>>>>>>     		}
>>>>>>     	} else {
>>>>>> +		if (!pmd_none(*vmf->pmd)) {
>>>>>> +			pte_t *ptep;
>>>>>> +
>>>>>> +			ptep = pte_offset_map(vmf->pmd, vmf->address);
>>>>>> +			if (unlikely(!ptep))
>>>>>> +				return VM_FAULT_NOPAGE;
>>>>>> +			/*
>>>>>> +			 * Recheck PTE as the PTE can be cleared temporarily
>>>>>> +			 * during a read+clear/modify/write update of the PTE,
>>>>>> +			 * eg, do_numa_page()/change_pte_range(). This will
>>>>>> +			 * trigger a major fault, even if we use mlockall,
>>>>>> +			 * which may affect performance.
>>>>>> +			 * We don't hold PTL here as acquiring PTL hurts
>>>>>> +			 * performance. So the check is still racy, but
>>>>>> +			 * the race window seems small enough.
>>>>>> +			 *
>>>>>> +			 * If we lose the race during the check, the page_fault
>>>>>> +			 * will be triggered. Butthe page table entry lock
>>>>>> +			 * still make sure the correctness:
>>>>>> +			 * - If the page cache is not reclaimed, the page_fault
>>>>>> +			 *   will work like the page fault was served already
>>>>>> +			 *   and bail out.
>>>>>> +			 * - If the page cache is reclaimed, the major fault
>>>>>> +			 *   will be triggered, page cache is filled,
>>>>>> +			 *   page_fault also work like the page fault was
>>>>>> +			 *   served already and bail out.
>>>>>> +			 */
>>>>> IMHO, this is too long.  It can be shorten to like,
>>>>>
>>>>> If we lose the race, major fault may be triggered unnecessary.  This
>>>>> hurts performance but not functionality.
>>>> OK, I'll fix it in the next version.
>>>>
>>>>>> +			if (unlikely(!pte_none(ptep_get_lockless(ptep))))
>>>>>> +				ret = VM_FAULT_NOPAGE;
>>>>>> +			pte_unmap(ptep);
>>>>>> +			if (unlikely(ret))
>>>>>> +				return ret;
>>>>>> +		}
>>>>>> +
>>>>>>     		/* No page in the page cache at all */
>>>>>>     		count_vm_event(PGMAJFAULT);
>>>>>>     		count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT);
>>>>> --
>>>>> Best Regards,
>>>>> Huang, Ying

-- 
Best Regards,
Peng

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