On 05.02.24 08:24, zhangpeng (AS) wrote:
> On 2024/2/5 14:52, Huang, Ying wrote:
>
>> "zhangpeng (AS)" <[email protected]> writes:
>>> On 2024/2/5 10:56, Huang, Ying wrote:
>>>> Peng Zhang <[email protected]> writes:
>>>>> 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 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/[email protected]/
>>>>> [2] https://github.com/antonblanchard/will-it-scale/
>>>>>
>>>>> Suggested-by: "Huang, Ying" <[email protected]>
>>>>> Suggested-by: Yin Fengwei <[email protected]>
>>>>> Signed-off-by: ZhangPeng <[email protected]>
>>>>> Signed-off-by: Kefeng Wang <[email protected]>
>>>>> ---
>>>>> 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.
--
Cheers,
David / dhildenb