Date: Thu, 29 Feb 2024 20:21:39 +0800
From: "zhangpeng (AS)" <zhangpeng362@...wei.com>
To: David Hildenbrand <david@...hat.com>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>
CC: <akpm@...ux-foundation.org>, <willy@...radead.org>,
	<ying.huang@...el.com>, <fengwei.yin@...el.com>,
	<aneesh.kumar@...ux.ibm.com>, <shy828301@...il.com>, <hughd@...gle.com>,
	<wangkefeng.wang@...wei.com>, <sunnanyong@...wei.com>
Subject: Re: [PATCH v3] filemap: avoid unnecessary major faults in
 filemap_fault()

On 2024/2/29 16:56, David Hildenbrand wrote:

> On 29.02.24 07:09, Peng Zhang wrote:
>> From: ZhangPeng <zhangpeng362@...wei.com>
>>
>> The major fault occurred when using mlockall(MCL_CURRENT | MCL_FUTURE)
>> in application, which leading to an unexpected issue[1].
>>
>> This caused by temporarily cleared PTE during a read+clear/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.
>>
>> This issue affects our traffic analysis service. The inbound traffic is
>> heavy. If a major fault occurs, the I/O schedule is triggered and the
>> original I/O is suspended. Generally, the I/O schedule is 0.7 ms. If
>> other applications are operating disks, the system needs to wait for
>> more than 10 ms. However, the inbound traffic is heavy and the NIC 
>> buffer
>> is small. As a result, packet loss occurs. But the traffic analysis 
>> service
>> can't tolerate packet loss.
>>
>> Fix this by holding PTL and rechecking the PTE in filemap_fault() before
>> triggering a major fault. We do this check only if vma is VM_LOCKED. In
>> our service test environment, the baseline is 7 major faults / 12 hours.
>> After the patch is applied, no major fault will be triggered.
>>
>> Testing file anonymous page read and write page fault performance in
>> ext4, tmpfs and ramdisk using will-it-scale[2] on a x86 physical 
>> machine.
>> The data is the average change compared with the mainline after the 
>> patch
>> is applied. The test results are indicates some performance regressions.
>> We do this check only if vma is VM_LOCKED, therefore, no performance
>> regressions is caused for most common cases.
>>
>> The test results are as follows:
>>                            processes processes_idle threads threads_idle
>> ext4    private file write: -0.51%    0.08%          -0.03% -0.04%
>> ext4    shared  file write:  0.135%  -0.531%          2.883% -0.772%
>> ramdisk private file write: -0.48%    0.23%          -1.08% 0.27%
>> ramdisk private file  read:  0.07%   -6.90%          -5.85% -0.70%
>> tmpfs   private file write: -0.344%  -0.110%          0.200% 0.145%
>> tmpfs   shared  file write:  0.958%   0.101%          2.781% -0.337%
>> tmpfs   private file  read: -0.16%    0.00%          -0.12% 0.41%
>>
>> [1] 
>> https://lore.kernel.org/linux-mm/9e62fd9a-bee0-52bf-50a7-498fa17434ee@huawei.com/
>> [2] https://github.com/antonblanchard/will-it-scale/
>>
>> Suggested-by: "Huang, Ying" <ying.huang@...el.com>
>> Suggested-by: David Hildenbrand <david@...hat.com>
>> Signed-off-by: ZhangPeng <zhangpeng362@...wei.com>
>> Signed-off-by: Kefeng Wang <wangkefeng.wang@...wei.com>
>> ---
>> v2->v3:
>> - Do this check only if vma is VM_LOCKED per David Hildenbrand
>> - Hold PTL and recheck the PTE
>> - Place the recheck code in a new function filemap_fault_recheck_pte()
>>
>> v1->v2:
>> - Add more test results per Huang, Ying
>> - Add more comments before check PTE per Huang, Ying, David Hildenbrand
>>    and Yin Fengwei
>> - Change pte_offset_map_nolock to pte_offset_map as the PTL won't
>>    be used
>>
>> RFC->v1:
>> - Add error handling when ptep == NULL per Huang, Ying and Matthew
>>    Wilcox
>> - Check the PTE without acquiring PTL in filemap_fault(), suggested by
>>    Huang, Ying and Yin Fengwei
>> - Add pmd_none() check before PTE map
>> - Update commit message and add performance test information
>>
>>   mm/filemap.c | 40 ++++++++++++++++++++++++++++++++++++++++
>>   1 file changed, 40 insertions(+)
>>
>> diff --git a/mm/filemap.c b/mm/filemap.c
>> index b4858d89f1b1..2668bac68df7 100644
>> --- a/mm/filemap.c
>> +++ b/mm/filemap.c
>> @@ -3181,6 +3181,42 @@ static struct file 
>> *do_async_mmap_readahead(struct vm_fault *vmf,
>>       return fpin;
>>   }
>>   +/*
>> + * filemap_fault_recheck_pte - hold PTL and recheck whether pte is 
>> none.
>> + * @vmf - the vm_fault for this fault.
>> + *
>> + * Recheck PTE as the PTE can be cleared temporarily during a 
>> read+clear/modify
>> + * /write update of the PTE, eg, do_numa_page()/change_pte_range(). 
>> This will
>> + * trigger an unexpected major fault, even if we use mlockall(), 
>> which may
>> + * increase IO and thus cause other unexpected behavior.
>> + *
>> + * Return VM_FAULT_NOPAGE if the PTE is not none or 
>> pte_offset_map_lock()
>> + * fails. In other cases, 0 is returned.
>> + */
>
> That documentation is imprecise, as you are not explaining the mlock 
> limitation.
>
> It's an internal helper, I'd drop all that and rather add a comment 
> below right next to the conditions that are performing the check ...
>
That makes sense.

>> +static vm_fault_t filemap_fault_recheck_pte(struct vm_fault *vmf)
>> +{
>> +    struct vm_area_struct *vma = vmf->vma;
>> +    vm_fault_t ret = 0;
>> +    pte_t *ptep;
>> +
>> +    if (!(vma->vm_flags & VM_LOCKED))
>> +        return ret;
>
> I was wondering if we also want to do:
>
> if (!is_cow_mappinng(vma->vm_flags))
>     return 0;
>
> But likely it's not helpful.
>
Maybe it's enough to check if the VMA is VM_LOCKED?
No performance degradation for most common scenarios.

>
> Then add something like:
>
> /*
>  * We might have COW'ed a pageache folio and might now have an mlocked

Nit: s/pageache/pagecache

>  * anon folio mapped. The original pagecache folio is not mlocked and
>  * might have been evicted. During a read+clear/modify/write update of
>  * the PTE, such as done in do_numa_page()/change_pte_range(), we
>  * temporarily clear the PTE under PT lock and might detect it here as
>  * "none" when not holding the PT lock.
>  *
>  * Not rechecking the PTE under PT lock could result in an
>  * unexpected major fault in an mlock'ed region. Recheck only for
>  * this special scenario while holding the PT lock, to not degrade
>  * non-mlocked scenarios.
>  */
>
Thanks!

I'll add these comments in the next version.

>> +
>> +    if (pmd_none(*vmf->pmd))
>> +        return ret;
>
> I'd  simply return 0 in both cases, easier to read.
>
Agreed.

>> +
>> +    ptep = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address,
>> +                   &vmf->ptl);
>> +    if (unlikely(!ptep))
>> +        return VM_FAULT_NOPAGE;
>> +
>> +    if (unlikely(!pte_none(ptep_get(ptep))))
>> +        ret = VM_FAULT_NOPAGE;
>> +
>> +    pte_unmap_unlock(ptep, vmf->ptl);
>> +    return ret;
>> +}
>
-- 
Best Regards,
Peng

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