[PATCH] filemap: avoid unnecessary major faults in filemap_fault()

[PATCH] filemap: avoid unnecessary major faults in filemap_fault()

[Peng Zhang]

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/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:
			processes processes_idle threads threads_idle
ext4 file write:	-1.14%    -0.08%         -1.87%  0.13%
ext4 file read:		 0.03%	  -0.65%         -0.51%	-0.08%
ramdisk file write:	-1.21%    -0.21%         -1.12%  0.11%
ramdisk 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@intel.com>
Suggested-by: Yin Fengwei <fengwei.yin@intel.com>
Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
---
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 | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/mm/filemap.c b/mm/filemap.c
index 142864338ca4..b29cdeb6a03b 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3238,6 +3238,24 @@ 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_nolock(vmf->vma->vm_mm, vmf->pmd,
+						     vmf->address, &vmf->ptl);
+			if (unlikely(!ptep))
+				return VM_FAULT_NOPAGE;
+			/*
+			 * Recheck pte as the pte can be cleared temporarily
+			 * during a read/modify/write update.
+			 */
+			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);
-- 
2.25.1

Re: [PATCH] filemap: avoid unnecessary major faults in filemap_fault()

[Huang, Ying]

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/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:
> 			processes processes_idle threads threads_idle
> ext4 file write:	-1.14%    -0.08%         -1.87%  0.13%
> ext4 file read:		 0.03%	  -0.65%         -0.51%	-0.08%
> ramdisk file write:	-1.21%    -0.21%         -1.12%  0.11%
> ramdisk file read:	 0.00%    -0.68%         -0.33% -0.02%

IIUC, this is the regression test results.  Right?  Can you also show
improvement test results to justify the change?

--
Best Regards,
Huang, Ying

> [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@intel.com>
> Suggested-by: Yin Fengwei <fengwei.yin@intel.com>
> Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> ---
> 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 | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 142864338ca4..b29cdeb6a03b 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -3238,6 +3238,24 @@ 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_nolock(vmf->vma->vm_mm, vmf->pmd,
> +						     vmf->address, &vmf->ptl);
> +			if (unlikely(!ptep))
> +				return VM_FAULT_NOPAGE;
> +			/*
> +			 * Recheck pte as the pte can be cleared temporarily
> +			 * during a read/modify/write update.
> +			 */
> +			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);

Re: [PATCH] filemap: avoid unnecessary major faults in filemap_fault()

[Huang, Ying]

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/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:
> 			processes processes_idle threads threads_idle
> ext4 file write:	-1.14%    -0.08%         -1.87%  0.13%
> ext4 file read:		 0.03%	  -0.65%         -0.51%	-0.08%
> ramdisk file write:	-1.21%    -0.21%         -1.12%  0.11%
> ramdisk 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@intel.com>
> Suggested-by: Yin Fengwei <fengwei.yin@intel.com>
> Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> ---
> 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 | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 142864338ca4..b29cdeb6a03b 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -3238,6 +3238,24 @@ 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_nolock(vmf->vma->vm_mm, vmf->pmd,
> +						     vmf->address, &vmf->ptl);
> +			if (unlikely(!ptep))
> +				return VM_FAULT_NOPAGE;
> +			/*
> +			 * Recheck pte as the pte can be cleared temporarily
> +			 * during a read/modify/write update.
> +			 */

I think that we should add some comments here about the racy checking.

--
Best Regards,
Huang, Ying

> +			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);

Re: [PATCH] filemap: avoid unnecessary major faults in filemap_fault()

[zhangpeng (AS)]

On 2024/2/5 10:54, 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/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:
>> 			processes processes_idle threads threads_idle
>> ext4 file write:	-1.14%    -0.08%         -1.87%  0.13%
>> ext4 file read:		 0.03%	  -0.65%         -0.51%	-0.08%
>> ramdisk file write:	-1.21%    -0.21%         -1.12%  0.11%
>> ramdisk file read:	 0.00%    -0.68%         -0.33% -0.02%
> IIUC, this is the regression test results.  Right?  Can you also show
> improvement test results to justify the change?

Sure, I'll add the improvement test results as follows:
			  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%

>
> --
> Best Regards,
> Huang, Ying
>
>> [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@intel.com>
>> Suggested-by: Yin Fengwei <fengwei.yin@intel.com>
>> Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>> ---
>> 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 | 18 ++++++++++++++++++
>>   1 file changed, 18 insertions(+)
>>
>> diff --git a/mm/filemap.c b/mm/filemap.c
>> index 142864338ca4..b29cdeb6a03b 100644
>> --- a/mm/filemap.c
>> +++ b/mm/filemap.c
>> @@ -3238,6 +3238,24 @@ 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_nolock(vmf->vma->vm_mm, vmf->pmd,
>> +						     vmf->address, &vmf->ptl);
>> +			if (unlikely(!ptep))
>> +				return VM_FAULT_NOPAGE;
>> +			/*
>> +			 * Recheck pte as the pte can be cleared temporarily
>> +			 * during a read/modify/write update.
>> +			 */
>> +			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,
Peng

Re: [PATCH] filemap: avoid unnecessary major faults in filemap_fault()

[zhangpeng (AS)]

On 2024/2/5 10:56, 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/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:
>> 			processes processes_idle threads threads_idle
>> ext4 file write:	-1.14%    -0.08%         -1.87%  0.13%
>> ext4 file read:		 0.03%	  -0.65%         -0.51%	-0.08%
>> ramdisk file write:	-1.21%    -0.21%         -1.12%  0.11%
>> ramdisk 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@intel.com>
>> Suggested-by: Yin Fengwei <fengwei.yin@intel.com>
>> Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>> ---
>> 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 | 18 ++++++++++++++++++
>>   1 file changed, 18 insertions(+)
>>
>> diff --git a/mm/filemap.c b/mm/filemap.c
>> index 142864338ca4..b29cdeb6a03b 100644
>> --- a/mm/filemap.c
>> +++ b/mm/filemap.c
>> @@ -3238,6 +3238,24 @@ 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_nolock(vmf->vma->vm_mm, vmf->pmd,
>> +						     vmf->address, &vmf->ptl);
>> +			if (unlikely(!ptep))
>> +				return VM_FAULT_NOPAGE;
>> +			/*
>> +			 * Recheck pte as the pte can be cleared temporarily
>> +			 * during a read/modify/write update.
>> +			 */
> I think that we should add some comments here about the racy checking.

I'll add comments in a v2 as follows:
/*
  * Recheck PTE as the PTE can be cleared temporarily
  * during a read/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.
  */

>
> --
> Best Regards,
> Huang, Ying
>
>> +			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,
Peng

Re: [PATCH] filemap: avoid unnecessary major faults in filemap_fault()

[Huang, Ying]

"zhangpeng (AS)" <zhangpeng362@huawei.com> writes:

> On 2024/2/5 10:56, 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/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:
>>> 			processes processes_idle threads threads_idle
>>> ext4 file write:	-1.14%    -0.08%         -1.87%  0.13%
>>> ext4 file read:		 0.03%	  -0.65%         -0.51%	-0.08%
>>> ramdisk file write:	-1.21%    -0.21%         -1.12%  0.11%
>>> ramdisk 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@intel.com>
>>> Suggested-by: Yin Fengwei <fengwei.yin@intel.com>
>>> Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
>>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>>> ---
>>> 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 | 18 ++++++++++++++++++
>>>   1 file changed, 18 insertions(+)
>>>
>>> diff --git a/mm/filemap.c b/mm/filemap.c
>>> index 142864338ca4..b29cdeb6a03b 100644
>>> --- a/mm/filemap.c
>>> +++ b/mm/filemap.c
>>> @@ -3238,6 +3238,24 @@ 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_nolock(vmf->vma->vm_mm, vmf->pmd,
>>> +						     vmf->address, &vmf->ptl);
>>> +			if (unlikely(!ptep))
>>> +				return VM_FAULT_NOPAGE;
>>> +			/*
>>> +			 * Recheck pte as the pte can be cleared temporarily
>>> +			 * during a read/modify/write update.
>>> +			 */
>> I think that we should add some comments here about the racy checking.
>
> I'll add comments in a v2 as follows:
> /*
>  * Recheck PTE as the PTE can be cleared temporarily
>  * during a read/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.
>  */

Sorry, my previous words aren't clear enough.  I mean some comments as
follows,

We don't hold PTL here, so the check is still racy.  But acquiring PTL
hurts performance and the race window seems small enough.

--
Best Regards,
Huang, Ying

>>> +			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);

Re: [PATCH] filemap: avoid unnecessary major faults in filemap_fault()

[zhangpeng (AS)]

On 2024/2/5 14:52, Huang, Ying wrote:

> "zhangpeng (AS)" <zhangpeng362@huawei.com> writes:
>> On 2024/2/5 10:56, 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/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:
>>>> 			processes processes_idle threads threads_idle
>>>> ext4 file write:	-1.14%    -0.08%         -1.87%  0.13%
>>>> ext4 file read:		 0.03%	  -0.65%         -0.51%	-0.08%
>>>> ramdisk file write:	-1.21%    -0.21%         -1.12%  0.11%
>>>> ramdisk 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@intel.com>
>>>> Suggested-by: Yin Fengwei <fengwei.yin@intel.com>
>>>> Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
>>>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>>>> ---
>>>> 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 | 18 ++++++++++++++++++
>>>>    1 file changed, 18 insertions(+)
>>>>
>>>> diff --git a/mm/filemap.c b/mm/filemap.c
>>>> index 142864338ca4..b29cdeb6a03b 100644
>>>> --- a/mm/filemap.c
>>>> +++ b/mm/filemap.c
>>>> @@ -3238,6 +3238,24 @@ 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_nolock(vmf->vma->vm_mm, vmf->pmd,
>>>> +						     vmf->address, &vmf->ptl);
>>>> +			if (unlikely(!ptep))
>>>> +				return VM_FAULT_NOPAGE;
>>>> +			/*
>>>> +			 * Recheck pte as the pte can be cleared temporarily
>>>> +			 * during a read/modify/write update.
>>>> +			 */
>>> I think that we should add some comments here about the racy checking.
>> I'll add comments in a v2 as follows:
>> /*
>>   * Recheck PTE as the PTE can be cleared temporarily
>>   * during a read/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.
>>   */
> Sorry, my previous words aren't clear enough.  I mean some comments as
> follows,
>
> We don't hold PTL here, so the check is still racy.  But acquiring PTL
> hurts performance and the race window seems small enough.

Got it. I'll add comments in a v2 as follows:
/*
  * Recheck PTE as the PTE can be cleared temporarily
  * during a read/modify/write update of the PTE.
  * We don't hold PTL here as acquiring PTL hurts
  * performance. So the check is still racy, but
  * the race window seems small enough.
  */

>
> --
> Best Regards,
> Huang, Ying
>
>>>> +			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,
Peng

Re: [PATCH] filemap: avoid unnecessary major faults in filemap_fault()

[David Hildenbrand]

On 05.02.24 08:24, zhangpeng (AS) wrote:
> On 2024/2/5 14:52, Huang, Ying wrote:
> 
>> "zhangpeng (AS)" <zhangpeng362@huawei.com> writes:
>>> On 2024/2/5 10:56, 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/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:
>>>>> 			processes processes_idle threads threads_idle
>>>>> ext4 file write:	-1.14%    -0.08%         -1.87%  0.13%
>>>>> ext4 file read:		 0.03%	  -0.65%         -0.51%	-0.08%
>>>>> ramdisk file write:	-1.21%    -0.21%         -1.12%  0.11%
>>>>> ramdisk 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@intel.com>
>>>>> Suggested-by: Yin Fengwei <fengwei.yin@intel.com>
>>>>> Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
>>>>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>>>>> ---
>>>>> 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 | 18 ++++++++++++++++++
>>>>>     1 file changed, 18 insertions(+)
>>>>>
>>>>> diff --git a/mm/filemap.c b/mm/filemap.c
>>>>> index 142864338ca4..b29cdeb6a03b 100644
>>>>> --- a/mm/filemap.c
>>>>> +++ b/mm/filemap.c
>>>>> @@ -3238,6 +3238,24 @@ 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_nolock(vmf->vma->vm_mm, vmf->pmd,
>>>>> +						     vmf->address, &vmf->ptl);
>>>>> +			if (unlikely(!ptep))
>>>>> +				return VM_FAULT_NOPAGE;
>>>>> +			/*
>>>>> +			 * Recheck pte as the pte can be cleared temporarily
>>>>> +			 * during a read/modify/write update.
>>>>> +			 */
>>>> I think that we should add some comments here about the racy checking.
>>> I'll add comments in a v2 as follows:
>>> /*
>>>    * Recheck PTE as the PTE can be cleared temporarily
>>>    * during a read/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.
>>>    */
>> Sorry, my previous words aren't clear enough.  I mean some comments as
>> follows,
>>
>> We don't hold PTL here, so the check is still racy.  But acquiring PTL
>> hurts performance and the race window seems small enough.
> 
> Got it. I'll add comments in a v2 as follows:
> /*
>    * Recheck PTE as the PTE can be cleared temporarily
>    * during a read/modify/write update of the PTE.
>    * We don't hold PTL here as acquiring PTL hurts
>    * performance. So the check is still racy, but
>    * the race window seems small enough.
>    */

It'd be worth spelling out what happens when we lose the race.

-- 
Cheers,

David / dhildenb

Re: [PATCH] filemap: avoid unnecessary major faults in filemap_fault()

[zhangpeng (AS)]

On 2024/2/5 15:31, David Hildenbrand wrote:

> On 05.02.24 08:24, zhangpeng (AS) wrote:
>> On 2024/2/5 14:52, Huang, Ying wrote:
>>
>>> "zhangpeng (AS)" <zhangpeng362@huawei.com> writes:
>>>> On 2024/2/5 10:56, 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/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:
>>>>>>             processes processes_idle threads threads_idle
>>>>>> ext4 file write:    -1.14%    -0.08%         -1.87% 0.13%
>>>>>> ext4 file read:         0.03%      -0.65% -0.51%    -0.08%
>>>>>> ramdisk file write:    -1.21%    -0.21%         -1.12% 0.11%
>>>>>> ramdisk 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@intel.com>
>>>>>> Suggested-by: Yin Fengwei <fengwei.yin@intel.com>
>>>>>> Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
>>>>>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>>>>>> ---
>>>>>> 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 | 18 ++++++++++++++++++
>>>>>>     1 file changed, 18 insertions(+)
>>>>>>
>>>>>> diff --git a/mm/filemap.c b/mm/filemap.c
>>>>>> index 142864338ca4..b29cdeb6a03b 100644
>>>>>> --- a/mm/filemap.c
>>>>>> +++ b/mm/filemap.c
>>>>>> @@ -3238,6 +3238,24 @@ 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_nolock(vmf->vma->vm_mm, vmf->pmd,
>>>>>> +                             vmf->address, &vmf->ptl);
>>>>>> +            if (unlikely(!ptep))
>>>>>> +                return VM_FAULT_NOPAGE;
>>>>>> +            /*
>>>>>> +             * Recheck pte as the pte can be cleared temporarily
>>>>>> +             * during a read/modify/write update.
>>>>>> +             */
>>>>> I think that we should add some comments here about the racy 
>>>>> checking.
>>>> I'll add comments in a v2 as follows:
>>>> /*
>>>>    * Recheck PTE as the PTE can be cleared temporarily
>>>>    * during a read/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.
>>>>    */
>>> Sorry, my previous words aren't clear enough.  I mean some comments as
>>> follows,
>>>
>>> We don't hold PTL here, so the check is still racy.  But acquiring PTL
>>> hurts performance and the race window seems small enough.
>>
>> Got it. I'll add comments in a v2 as follows:
>> /*
>>    * Recheck PTE as the PTE can be cleared temporarily
>>    * during a read/modify/write update of the PTE.
>>    * We don't hold PTL here as acquiring PTL hurts
>>    * performance. So the check is still racy, but
>>    * the race window seems small enough.
>>    */
>
> It'd be worth spelling out what happens when we lose the race.
>
I'll add what happens when we lose the race as follows:
/*
  * Recheck PTE as the PTE can be cleared temporarily
  * during a read/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.
  */

-- 
Best Regards,
Peng

Re: [PATCH] filemap: avoid unnecessary major faults in filemap_fault()

[Yin Fengwei]

On 2/5/24 15:36, zhangpeng (AS) wrote:
> On 2024/2/5 15:31, David Hildenbrand wrote:
> 
>> On 05.02.24 08:24, zhangpeng (AS) wrote:
>>> On 2024/2/5 14:52, Huang, Ying wrote:
>>>
>>>> "zhangpeng (AS)" <zhangpeng362@huawei.com> writes:
>>>>> On 2024/2/5 10:56, 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/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:
>>>>>>>             processes processes_idle threads threads_idle
>>>>>>> ext4 file write:    -1.14%    -0.08%         -1.87% 0.13%
>>>>>>> ext4 file read:         0.03%      -0.65% -0.51%    -0.08%
>>>>>>> ramdisk file write:    -1.21%    -0.21%         -1.12% 0.11%
>>>>>>> ramdisk 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@intel.com>
>>>>>>> Suggested-by: Yin Fengwei <fengwei.yin@intel.com>
>>>>>>> Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
>>>>>>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>>>>>>> ---
>>>>>>> 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 | 18 ++++++++++++++++++
>>>>>>>     1 file changed, 18 insertions(+)
>>>>>>>
>>>>>>> diff --git a/mm/filemap.c b/mm/filemap.c
>>>>>>> index 142864338ca4..b29cdeb6a03b 100644
>>>>>>> --- a/mm/filemap.c
>>>>>>> +++ b/mm/filemap.c
>>>>>>> @@ -3238,6 +3238,24 @@ 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_nolock(vmf->vma->vm_mm, vmf->pmd,
>>>>>>> +                             vmf->address, &vmf->ptl);
>>>>>>> +            if (unlikely(!ptep))
>>>>>>> +                return VM_FAULT_NOPAGE;
>>>>>>> +            /*
>>>>>>> +             * Recheck pte as the pte can be cleared temporarily
>>>>>>> +             * during a read/modify/write update.
>>>>>>> +             */
>>>>>> I think that we should add some comments here about the racy checking.
>>>>> I'll add comments in a v2 as follows:
>>>>> /*
>>>>>    * Recheck PTE as the PTE can be cleared temporarily
>>>>>    * during a read/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.
>>>>>    */
>>>> Sorry, my previous words aren't clear enough.  I mean some comments as
>>>> follows,
>>>>
>>>> We don't hold PTL here, so the check is still racy.  But acquiring PTL
>>>> hurts performance and the race window seems small enough.
>>>
>>> Got it. I'll add comments in a v2 as follows:
>>> /*
>>>    * Recheck PTE as the PTE can be cleared temporarily
>>>    * during a read/modify/write update of the PTE.
>>>    * We don't hold PTL here as acquiring PTL hurts
>>>    * performance. So the check is still racy, but
>>>    * the race window seems small enough.
>>>    */
>>
>> It'd be worth spelling out what happens when we lose the race.
>>
> I'll add what happens when we lose the race as follows:
> /*
>  * Recheck PTE as the PTE can be cleared temporarily
>  * during a read/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.
>  */
> 

I believe David was asking to add:

"...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.
"

Re: [PATCH] filemap: avoid unnecessary major faults in filemap_fault()

[zhangpeng (AS)]

On 2024/2/5 16:40, Yin Fengwei wrote:

>
> On 2/5/24 15:36, zhangpeng (AS) wrote:
>> On 2024/2/5 15:31, David Hildenbrand wrote:
>>
>>> On 05.02.24 08:24, zhangpeng (AS) wrote:
>>>> On 2024/2/5 14:52, Huang, Ying wrote:
>>>>
>>>>> "zhangpeng (AS)" <zhangpeng362@huawei.com> writes:
>>>>>> On 2024/2/5 10:56, 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/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:
>>>>>>>>              processes processes_idle threads threads_idle
>>>>>>>> ext4 file write:    -1.14%    -0.08%         -1.87% 0.13%
>>>>>>>> ext4 file read:         0.03%      -0.65% -0.51%    -0.08%
>>>>>>>> ramdisk file write:    -1.21%    -0.21%         -1.12% 0.11%
>>>>>>>> ramdisk 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@intel.com>
>>>>>>>> Suggested-by: Yin Fengwei <fengwei.yin@intel.com>
>>>>>>>> Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
>>>>>>>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>>>>>>>> ---
>>>>>>>> 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 | 18 ++++++++++++++++++
>>>>>>>>      1 file changed, 18 insertions(+)
>>>>>>>>
>>>>>>>> diff --git a/mm/filemap.c b/mm/filemap.c
>>>>>>>> index 142864338ca4..b29cdeb6a03b 100644
>>>>>>>> --- a/mm/filemap.c
>>>>>>>> +++ b/mm/filemap.c
>>>>>>>> @@ -3238,6 +3238,24 @@ 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_nolock(vmf->vma->vm_mm, vmf->pmd,
>>>>>>>> +                             vmf->address, &vmf->ptl);
>>>>>>>> +            if (unlikely(!ptep))
>>>>>>>> +                return VM_FAULT_NOPAGE;
>>>>>>>> +            /*
>>>>>>>> +             * Recheck pte as the pte can be cleared temporarily
>>>>>>>> +             * during a read/modify/write update.
>>>>>>>> +             */
>>>>>>> I think that we should add some comments here about the racy checking.
>>>>>> I'll add comments in a v2 as follows:
>>>>>> /*
>>>>>>     * Recheck PTE as the PTE can be cleared temporarily
>>>>>>     * during a read/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.
>>>>>>     */
>>>>> Sorry, my previous words aren't clear enough.  I mean some comments as
>>>>> follows,
>>>>>
>>>>> We don't hold PTL here, so the check is still racy.  But acquiring PTL
>>>>> hurts performance and the race window seems small enough.
>>>> Got it. I'll add comments in a v2 as follows:
>>>> /*
>>>>     * Recheck PTE as the PTE can be cleared temporarily
>>>>     * during a read/modify/write update of the PTE.
>>>>     * We don't hold PTL here as acquiring PTL hurts
>>>>     * performance. So the check is still racy, but
>>>>     * the race window seems small enough.
>>>>     */
>>> It'd be worth spelling out what happens when we lose the race.
>>>
>> I'll add what happens when we lose the race as follows:
>> /*
>>   * Recheck PTE as the PTE can be cleared temporarily
>>   * during a read/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.
>>   */
>>
> I believe David was asking to add:
>
> "...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.
> "

Got it.
Thanks!

-- 
Best Regards,
Peng