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

[Huang, Ying]

"Huang, Ying" <ying.huang@xxxxxxxxx> writes:

> "zhangpeng (AS)" <zhangpeng362@xxxxxxxxxx> writes:
>
>> On 2023/11/23 13:26, Yin Fengwei wrote:
>>
>>> On 11/23/23 12:12, zhangpeng (AS) wrote:
>>>> On 2023/11/23 9:09, Yin Fengwei wrote:
>>>>
>>>>> Hi Peng,
>>>>>
>>>>> On 11/22/23 22:00, Peng Zhang wrote:
>>>>>> 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>
>>>>>> ---
>>>>>>    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;
>>>>>> +        }
>>>>> I am curious. Did you try not to take PTL here and just check whether PTE is not NONE?
>>>> Thank you for your reply.
>>>>
>>>> If we don't take PTL, the current use case won't trigger this issue either.
>>> Is this verified by testing or just in theory?
>>
>> If we add a delay between ptep_modify_prot_start() and ptep_modify_prot_commit(),
>> this issue will also trigger. Without delay, we haven't reproduced this problem
>> so far.
>>
>>>> In most cases, if we don't take PTL, this issue won't be triggered. However,
>>>> there is still a possibility of triggering this issue. The corner case is that
>>>> task 2 triggers a page fault when task 1 is between ptep_modify_prot_start()
>>>> and ptep_modify_prot_commit() in do_numa_page(). Furthermore,task 2 passes the
>>>> check whether the PTE is not NONE before task 1 updates PTE in
>>>> ptep_modify_prot_commit() without taking PTL.
>>> There is very limited operations between ptep_modify_prot_start() and
>>> ptep_modify_prot_commit(). While the code path from page fault to this check is
>>> long. My understanding is it's very likely the PTE is not NONE when do PTE check
>>> here without hold PTL (This is my theory. :)).
>>
>> Yes, there is a high probability that this issue won't occur without taking PTL.
>>
>>> In the other side, acquiring/releasing PTL may bring performance impaction. It may
>>> not be big deal because the IO operations in this code path. But it's better to
>>> collect some performance data IMHO.
>>
>> We tested the performance of file private mapping page fault (page_fault2.c of
>> will-it-scale [1]) and file shared mapping page fault (page_fault3.c of will-it-scale).
>> The difference in performance (in operations per second) before and after patch
>> applied is about 0.7% on a x86 physical machine.
>
> Whether is it improvement or reduction?

And I think that you need to test ramdisk cases too to verify whether
this will cause performance regression and how much.

--
Best Regards,
Huang, Ying

> --
> Best Regards,
> Huang, Ying
>
>> [1] https://github.com/antonblanchard/will-it-scale/tree/master
>>
>>>
>>> Regards
>>> Yin, Fengwei
>>>
>>>>> Regards
>>>>> Yin, Fengwei
>>>>>
>>>>>> +
>>>>>>            /* No page in the page cache at all */
>>>>>>            count_vm_event(PGMAJFAULT);
>>>>>>            count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT);