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

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

On 2024/2/5 16:40, Yin Fengwei wrote:

>
> On 2/5/24 15:36, zhangpeng (AS) wrote:
>> 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@huawei.com> writes:
>>>>>> On 2024/2/5 10:56, Huang, Ying wrote:
>>>>>>> Peng Zhang <zhangpeng362@huawei.com> writes:
>>>>>>>> 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 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@intel.com>
>>>>>>>> Suggested-by: Yin Fengwei <fengwei.yin@intel.com>
>>>>>>>> Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
>>>>>>>> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.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.
>>   */
>>
> I believe David was asking to add:
>
> "...but the race window seems small enough.
>
>   If we lose the race during the check, the page_fault will
>   be triggered. Butthe page table entry lock still make sure
>   the correctness:
>     - If the page cache is not reclaimed, the page_fault will
>       work like the page fault was served already and bail out.
>     - If the page cache is reclaimed, the major fault will be
>       triggered, page cache is filled, page_fault also work
>       like the page fault was served already and bail out.
> "

Got it.
Thanks!

-- 
Best Regards,
Peng