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

[Huang, Ying]

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

> On 2023/11/23 16:36, 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 by holding ptl in filemap_fault() before
>>> triggering a major fault.
>>>
>>> [1] https://lore.kernel.org/linux-mm/9e62fd9a-bee0-52bf-50a7-498fa17434ee@xxxxxxxxxx/
>>>
>>> Signed-off-by: ZhangPeng <zhangpeng362@xxxxxxxxxx>
>>> Signed-off-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
>> Suggested-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
>>
>> :-)
>>
>>> ---
>>>   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.
>
> Maybe we don't need to deal with ptep == NULL, because it has been
> considered later in filemap_fault()?
> ptep == NULL means that the ptep has been replaced with a PMD entry.
> In this case, major fault is also required.

I still think that we need to deal with that.  That is common error
processing logic.

--
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);