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

[David Hildenbrand <david@redhat.com>]

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