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

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

On 2023/11/24 12:26, Huang, Ying wrote:

> "Huang, Ying" <ying.huang@intel.com> writes:
>
>> "zhangpeng (AS)" <zhangpeng362@huawei.com> 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@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>
>>>>>>> ---
>>>>>>>     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? 😁

> --
> 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);

-- 
Best Regards,
Peng