linux-kernel - Re: [PATCH] filemap: avoid unnecessary major faults in filemap

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Date: Mon, 5 Feb 2024 14:43:07 +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>
Subject: Re: [PATCH] filemap: avoid unnecessary major faults in
 filemap_fault()

On 2024/2/5 10:56, 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/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@...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>
>> ---
>> 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