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

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

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