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

[Huang, Ying]

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

> On 2023/11/24 12:26, Huang, Ying wrote:
>
>> "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.
>
> Yes, I will.
> In addition, are there any ramdisk test cases recommended? 😁

I think that you can start with the will-it-scale test case you used
before.  And you can try some workload with large number of major fault,
like file read with mmap.

--
Best Regards,
Huang, Ying

>> --
>> 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);