Re: [PATCH v3] filemap: avoid unnecessary major faults in filemap_fault()

["zhangpeng (AS)" <zhangpeng362@huawei.com>]

On 2024/2/29 14:31, 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 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.
>>
>> 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. But the traffic analysis service
>> can't tolerate packet loss.
>>
>> Fix this by holding PTL and rechecking the PTE in filemap_fault() before
>> triggering a major fault. We do this check only if vma is VM_LOCKED. In
>> our service test environment, the baseline is 7 major faults / 12 hours.
>> After the patch is applied, no major fault will be triggered.
>>
>> 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. The test results are indicates some performance regressions.
>> We do this check only if vma is VM_LOCKED, therefore, no performance
>> regressions is caused for most common cases.
>>
>> The test results are as follows:
>>                            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%
> Have you retested with the VM_LOCKED optimization?  Why are there still
> performance regression?
>
>> 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%
>>
>> [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: David Hildenbrand <david@redhat.com>
>> Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
>> ---
>> v2->v3:
>> - Do this check only if vma is VM_LOCKED per David Hildenbrand
>> - Hold PTL and recheck the PTE
>> - Place the recheck code in a new function filemap_fault_recheck_pte()
>>
>> 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 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 | 40 ++++++++++++++++++++++++++++++++++++++++
>>   1 file changed, 40 insertions(+)
>>
>> diff --git a/mm/filemap.c b/mm/filemap.c
>> index b4858d89f1b1..2668bac68df7 100644
>> --- a/mm/filemap.c
>> +++ b/mm/filemap.c
>> @@ -3181,6 +3181,42 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf,
>>   	return fpin;
>>   }
>>   
>> +/*
>> + * filemap_fault_recheck_pte - hold PTL and recheck whether pte is none.
>> + * @vmf - the vm_fault for this fault.
>> + *
>> + * 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 an unexpected major fault, even if we use mlockall(), which may
>> + * increase IO and thus cause other unexpected behavior.
>> + *
>> + * Return VM_FAULT_NOPAGE if the PTE is not none or pte_offset_map_lock()
>> + * fails. In other cases, 0 is returned.
>> + */
>> +static vm_fault_t filemap_fault_recheck_pte(struct vm_fault *vmf)
>> +{
>> +	struct vm_area_struct *vma = vmf->vma;
>> +	vm_fault_t ret = 0;
>> +	pte_t *ptep;
>> +
>> +	if (!(vma->vm_flags & VM_LOCKED))
>> +		return ret;
>> +
>> +	if (pmd_none(*vmf->pmd))
>> +		return ret;
>> +
> How about check PTE without lock firstly?  I guess that this can improve
> performance in common case (no race).

That's a good idea. IIUC, we can check PTE without lock firstly before checking VM_LOCKED,
thus reducing the number of times we hold the PTL.

>> +	ptep = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address,
>> +				   &vmf->ptl);
>> +	if (unlikely(!ptep))
>> +		return VM_FAULT_NOPAGE;
>> +
>> +	if (unlikely(!pte_none(ptep_get(ptep))))
>> +		ret = VM_FAULT_NOPAGE;
>> +
>> +	pte_unmap_unlock(ptep, vmf->ptl);
>> +	return ret;
>> +}
>> +
>>   /**
>>    * filemap_fault - read in file data for page fault handling
>>    * @vmf:	struct vm_fault containing details of the fault
>> @@ -3236,6 +3272,10 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
>>   			mapping_locked = true;
>>   		}
>>   	} else {
>> +		ret = filemap_fault_recheck_pte(vmf);
>> +		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