Re: [Question]: major faults are still triggered after mlockall when numa balancing

[Aneesh Kumar K V <aneesh.kumar@linux.ibm.com>]

On 11/10/23 9:20 AM, Yin, Fengwei wrote:
> 
> 
> On 11/10/2023 11:39 AM, Kefeng Wang wrote:
>>
>>
>> On 2023/11/10 9:57, Yin, Fengwei wrote:
>>>
>>>
>>> On 11/10/2023 6:54 AM, Yang Shi wrote:
>>>> On Thu, Nov 9, 2023 at 5:48 AM zhangpeng (AS) <zhangpeng362@huawei.com> wrote:
>>>>>
>>>>> Hi everyone,
>>>>>
>>>>> There is a performance issue that has been bothering us recently.
>>>>> This problem can reproduce in the latest mainline version (Linux 6.6).
>>>>>
>>>>> We use mlockall(MCL_CURRENT | MCL_FUTURE) in the user mode process
>>>>> to avoid performance problems caused by major fault.
>>>>>
>>>>> There is a stage in numa fault which will set pte as 0 in do_numa_page() :
>>>>> ptep_modify_prot_start() will clear the vmf->pte, until
>>>>> ptep_modify_prot_commit() assign a value to the vmf->pte.
>>>>>
>>>>> 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 zero which is concurrently set by 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.
>>>>>
>>>>> Our problem scenario is as follows:
>>>>>
>>>>> task 1                      task 2
>>>>> ------                      ------
>>>>> /* scan global variables */
>>>>> do_numa_page()
>>>>>     spin_lock(vmf->ptl)
>>>>>     ptep_modify_prot_start()
>>>>>     /* set vmf->pte as null */
>>>>>                               /* Access global variables */
>>>>>                               handle_pte_fault()
>>>>>                                 /* no pte lock */
>>>>>                                 do_pte_missing()
>>>>>                                   do_fault()
>>>>>                                     do_read_fault()
>>>>>     ptep_modify_prot_commit()
>>>>>     /* ptep update done */
>>>>>     pte_unmap_unlock(vmf->pte, vmf->ptl)
>>>>>                                       do_fault_around()
>>>>>                                       __do_fault()
>>>>>                                         filemap_fault()
>>>>>                                           /* page cache is not available
>>>>>                                           and a major fault is triggered */
>>>>>                                           do_sync_mmap_readahead()
>>>>>                                           /* page_not_uptodate and goto
>>>>>                                           out_retry. */
>>>>>
>>>>> Is there any way to avoid such a major fault?
>>>>
>>>> IMHO I don't think it is a bug. The man page quoted by Willy says "All
>>>> mapped pages are guaranteed to be resident in RAM when the call
>>>> returns successfully", but the later COW already made the file page
>>>> unmapped, right? The PTE pointed to the COW'ed anon page.
>>>> Hypothetically if we kept the file page mlocked and unmapped,
>>>> munlock() would have not munlocked the file page at all, it would be
>>>> mlocked in memory forever.
>>> But in this case, even the COW page is mlocked. There is small window
>>> that PTE is set to null in do_numa_page(). data segment access (it's to
>>> COW page which has nothing to do with original page cache) happens in
>>> this small window will trigger filemap_fault() to fault in original
>>> page cache.
>>>
>>> I had thought to do double check whether vmf->pte is NULL in do_read_fault().
>>> But it's not reliable enough.
>>>
>>> Matthew's idea to use protnone to block both hardware accessing and
>>> do_pte_missing() looks more promising to me.
>>
>> Actual， we could revert the following patch to avoid this issue,
>> but this workaroud from ppc...
>>
>> commit cee216a696b2004017a5ecb583366093d90b1568
>> Author: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
>> Date:   Fri Feb 24 14:59:13 2017 -0800
>>
>>     mm/autonuma: don't use set_pte_at when updating protnone ptes
>>
>>     Architectures like ppc64, use privilege access bit to mark pte non
>>     accessible.  This implies that kernel can do a copy_to_user to an
>>     address marked for numa fault.  This also implies that there can be a
>>     parallel hardware update for the pte.  set_pte_at cannot be used in such
>>     scenarios.  Hence switch the pte update to use ptep_get_and_clear and
>>     set_pte_at combination.
> Oh. This means the protnone doesn't work for PPC.
> 
>

That is correct. I am yet to read the full thread. Can we make ptep_modify_prot_start()
not to mark pte = 0 ? One of the requirement for powerpc is to mark it hardware invalid
such that not TLB entries get inserted after that. Other options is to get a proper
pte_update API for generic kernel so that architectures can do this without marking the
pte invalid. 


-aneesh