"zhangpeng (AS)" <[email protected]> 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 <[email protected]>
>>>>>
>>>>> 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/[email protected]/
>>>>>
>>>>> Signed-off-by: ZhangPeng <[email protected]>
>>>>> Signed-off-by: Kefeng Wang <[email protected]>
>>>>> ---
>>>>> 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?
--
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);
"Huang, Ying" <[email protected]> writes:
> "zhangpeng (AS)" <[email protected]> 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 <[email protected]>
>>>>>>
>>>>>> 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/[email protected]/
>>>>>>
>>>>>> Signed-off-by: ZhangPeng <[email protected]>
>>>>>> Signed-off-by: Kefeng Wang <[email protected]>
>>>>>> ---
>>>>>> 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);