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

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

On 2023/11/23 16:36, 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 by holding ptl in filemap_fault() before
>> triggering a major fault.
>>
>> [1] https://lore.kernel.org/linux-mm/9e62fd9a-bee0-52bf-50a7-498fa17434ee@huawei.com/
>>
>> Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> Suggested-by: "Huang, Ying" <ying.huang@intel.com>
>
> :-)

Yes! :-)

>> ---
>>   mm/filemap.c | 14 ++++++++++++++
>>   1 file changed, 14 insertions(+)
>>
>> diff --git a/mm/filemap.c b/mm/filemap.c
>> index 71f00539ac00..bb5e6a2790dc 100644
>> --- a/mm/filemap.c
>> +++ b/mm/filemap.c
>> @@ -3226,6 +3226,20 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
>>   			mapping_locked = true;
>>   		}
>>   	} else {
>> +		pte_t *ptep = pte_offset_map_lock(vmf->vma->vm_mm, vmf->pmd,
>> +						  vmf->address, &vmf->ptl);
>> +		if (ptep) {
>> +			/*
>> +			 * Recheck pte with ptl locked as the pte can be cleared
>> +			 * temporarily during a read/modify/write update.
>> +			 */
>> +			if (unlikely(!pte_none(ptep_get(ptep))))
>> +				ret = VM_FAULT_NOPAGE;
>> +			pte_unmap_unlock(ptep, vmf->ptl);
>> +			if (unlikely(ret))
>> +				return ret;
>> +		}
>> +
> Need to deal with ptep == NULL.  Although that is high impossible.

If ptep == NULL, we may just need to return VM_FAULT_SIGBUS.
I'll add it in the next version.

Thanks!

> --
> Best Regards,
> Huang, Ying
>
>>   		/* No page in the page cache at all */
>>   		count_vm_event(PGMAJFAULT);
>>   		count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT);

-- 
Best Regards,
Peng