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

[Huang, Ying]

"zhangpeng (AS)" <zhangpeng362@xxxxxxxxxx> writes:

> On 2024/2/5 10:56, Huang, Ying wrote:
>
>> Peng Zhang <zhangpeng362@xxxxxxxxxx> writes:
>>> From: ZhangPeng <zhangpeng362@xxxxxxxxxx>
>>>
>>> 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@xxxxxxxxxx/
>>> [2] https://github.com/antonblanchard/will-it-scale/
>>>
>>> Suggested-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
>>> Suggested-by: Yin Fengwei <fengwei.yin@xxxxxxxxx>
>>> Signed-off-by: ZhangPeng <zhangpeng362@xxxxxxxxxx>
>>> Signed-off-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
>>> ---
>>> 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);