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Date: Mon, 5 Feb 2024 15:36:01 +0800
From: "zhangpeng (AS)" <zhangpeng362@...wei.com>
To: David Hildenbrand <david@...hat.com>, "Huang, Ying" <ying.huang@...el.com>
CC: <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
<akpm@...ux-foundation.org>, <willy@...radead.org>, <fengwei.yin@...el.com>,
<aneesh.kumar@...ux.ibm.com>, <shy828301@...il.com>, <hughd@...gle.com>,
<wangkefeng.wang@...wei.com>
Subject: Re: [PATCH] filemap: avoid unnecessary major faults in
filemap_fault()
On 2024/2/5 15:31, David Hildenbrand wrote:
> On 05.02.24 08:24, zhangpeng (AS) wrote:
>> On 2024/2/5 14:52, Huang, Ying wrote:
>>
>>> "zhangpeng (AS)" <zhangpeng362@...wei.com> writes:
>>>> On 2024/2/5 10:56, Huang, Ying wrote:
>>>>> Peng Zhang <zhangpeng362@...wei.com> writes:
>>>>>> From: ZhangPeng <zhangpeng362@...wei.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 without acquiring PTL in
>>>>>> filemap_fault()
>>>>>> before triggering a major fault.
>>>>>>
>>>>>> Testing file anonymous page read and write page fault performance
>>>>>> in ext4
>>>>>> 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 within the range of fluctuation,
>>>>>> and there
>>>>>> is no obvious difference. The test results are as follows:
>>>>>> processes processes_idle threads threads_idle
>>>>>> ext4 file write: -1.14% -0.08% -1.87% 0.13%
>>>>>> ext4 file read: 0.03% -0.65% -0.51% -0.08%
>>>>>> ramdisk file write: -1.21% -0.21% -1.12% 0.11%
>>>>>> ramdisk file read: 0.00% -0.68% -0.33% -0.02%
>>>>>>
>>>>>> [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: Yin Fengwei <fengwei.yin@...el.com>
>>>>>> Signed-off-by: ZhangPeng <zhangpeng362@...wei.com>
>>>>>> Signed-off-by: Kefeng Wang <wangkefeng.wang@...wei.com>
>>>>>> ---
>>>>>> 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 | 18 ++++++++++++++++++
>>>>>> 1 file changed, 18 insertions(+)
>>>>>>
>>>>>> diff --git a/mm/filemap.c b/mm/filemap.c
>>>>>> index 142864338ca4..b29cdeb6a03b 100644
>>>>>> --- a/mm/filemap.c
>>>>>> +++ b/mm/filemap.c
>>>>>> @@ -3238,6 +3238,24 @@ vm_fault_t filemap_fault(struct vm_fault
>>>>>> *vmf)
>>>>>> mapping_locked = true;
>>>>>> }
>>>>>> } else {
>>>>>> + if (!pmd_none(*vmf->pmd)) {
>>>>>> + pte_t *ptep;
>>>>>> +
>>>>>> + ptep = pte_offset_map_nolock(vmf->vma->vm_mm, vmf->pmd,
>>>>>> + vmf->address, &vmf->ptl);
>>>>>> + if (unlikely(!ptep))
>>>>>> + return VM_FAULT_NOPAGE;
>>>>>> + /*
>>>>>> + * Recheck pte as the pte can be cleared temporarily
>>>>>> + * during a read/modify/write update.
>>>>>> + */
>>>>> I think that we should add some comments here about the racy
>>>>> checking.
>>>> I'll add comments in a v2 as follows:
>>>> /*
>>>> * Recheck PTE as the PTE can be cleared temporarily
>>>> * during a read/modify/write update of the PTE, eg,
>>>> * do_numa_page()/change_pte_range(). This will trigger
>>>> * a major fault, even if we use mlockall, which may
>>>> * affect performance.
>>>> */
>>> Sorry, my previous words aren't clear enough. I mean some comments as
>>> follows,
>>>
>>> We don't hold PTL here, so the check is still racy. But acquiring PTL
>>> hurts performance and the race window seems small enough.
>>
>> Got it. I'll add comments in a v2 as follows:
>> /*
>> * Recheck PTE as the PTE can be cleared temporarily
>> * during a read/modify/write update of the PTE.
>> * We don't hold PTL here as acquiring PTL hurts
>> * performance. So the check is still racy, but
>> * the race window seems small enough.
>> */
>
> It'd be worth spelling out what happens when we lose the race.
>
I'll add what happens when we lose the race as follows:
/*
* Recheck PTE as the PTE can be cleared temporarily
* during a read/modify/write update of the PTE, eg,
* do_numa_page()/change_pte_range(). This will trigger
* a major fault, even if we use mlockall, which may
* affect performance.
* We don't hold PTL here as acquiring PTL hurts
* performance. So the check is still racy, but
* the race window seems small enough.
*/
--
Best Regards,
Peng
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