"zhangpeng (AS)" <zhangpeng362@xxxxxxxxxx> writes: > On 2023/11/24 12:26, Huang, Ying wrote: > >> "Huang, Ying" <ying.huang@xxxxxxxxx> writes: >> >>> "zhangpeng (AS)" <zhangpeng362@xxxxxxxxxx> writes: >>> >>>> On 2023/11/23 13:26, Yin Fengwei wrote: >>>> >>>>> On 11/23/23 12:12, zhangpeng (AS) wrote: >>>>>> On 2023/11/23 9:09, Yin Fengwei wrote: >>>>>> >>>>>>> Hi Peng, >>>>>>> >>>>>>> On 11/22/23 22:00, Peng Zhang wrote: >>>>>>>> From: ZhangPeng <zhangpeng362@xxxxxxxxxx> >>>>>>>> >>>>>>>> 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 by holding ptl in filemap_fault() before >>>>>>>> triggering a major fault. >>>>>>>> >>>>>>>> [1] https://lore.kernel.org/linux-mm/9e62fd9a-bee0-52bf-50a7-498fa17434ee@xxxxxxxxxx/ >>>>>>>> >>>>>>>> Signed-off-by: ZhangPeng <zhangpeng362@xxxxxxxxxx> >>>>>>>> Signed-off-by: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx> >>>>>>>> --- >>>>>>>> mm/filemap.c | 14 ++++++++++++++ >>>>>>>> 1 file changed, 14 insertions(+) >>>>>>>> >>>>>>>> diff --git a/mm/filemap.c b/mm/filemap.c >>>>>>>> index 71f00539ac00..bb5e6a2790dc 100644 >>>>>>>> --- a/mm/filemap.c >>>>>>>> +++ b/mm/filemap.c >>>>>>>> @@ -3226,6 +3226,20 @@ vm_fault_t filemap_fault(struct vm_fault *vmf) >>>>>>>> mapping_locked = true; >>>>>>>> } >>>>>>>> } else { >>>>>>>> + pte_t *ptep = pte_offset_map_lock(vmf->vma->vm_mm, vmf->pmd, >>>>>>>> + vmf->address, &vmf->ptl); >>>>>>>> + if (ptep) { >>>>>>>> + /* >>>>>>>> + * Recheck pte with ptl locked as the pte can be cleared >>>>>>>> + * temporarily during a read/modify/write update. >>>>>>>> + */ >>>>>>>> + if (unlikely(!pte_none(ptep_get(ptep)))) >>>>>>>> + ret = VM_FAULT_NOPAGE; >>>>>>>> + pte_unmap_unlock(ptep, vmf->ptl); >>>>>>>> + if (unlikely(ret)) >>>>>>>> + return ret; >>>>>>>> + } >>>>>>> I am curious. Did you try not to take PTL here and just check whether PTE is not NONE? >>>>>> Thank you for your reply. >>>>>> >>>>>> If we don't take PTL, the current use case won't trigger this issue either. >>>>> Is this verified by testing or just in theory? >>>> If we add a delay between ptep_modify_prot_start() and ptep_modify_prot_commit(), >>>> this issue will also trigger. Without delay, we haven't reproduced this problem >>>> so far. >>>> >>>>>> In most cases, if we don't take PTL, this issue won't be triggered. However, >>>>>> there is still a possibility of triggering this issue. The corner case is that >>>>>> task 2 triggers a page fault when task 1 is between ptep_modify_prot_start() >>>>>> and ptep_modify_prot_commit() in do_numa_page(). Furthermore,task 2 passes the >>>>>> check whether the PTE is not NONE before task 1 updates PTE in >>>>>> ptep_modify_prot_commit() without taking PTL. >>>>> There is very limited operations between ptep_modify_prot_start() and >>>>> ptep_modify_prot_commit(). While the code path from page fault to this check is >>>>> long. My understanding is it's very likely the PTE is not NONE when do PTE check >>>>> here without hold PTL (This is my theory. :)). >>>> Yes, there is a high probability that this issue won't occur without taking PTL. >>>> >>>>> In the other side, acquiring/releasing PTL may bring performance impaction. It may >>>>> not be big deal because the IO operations in this code path. But it's better to >>>>> collect some performance data IMHO. >>>> We tested the performance of file private mapping page fault (page_fault2.c of >>>> will-it-scale [1]) and file shared mapping page fault (page_fault3.c of will-it-scale). >>>> The difference in performance (in operations per second) before and after patch >>>> applied is about 0.7% on a x86 physical machine. >>> Whether is it improvement or reduction? >> And I think that you need to test ramdisk cases too to verify whether >> this will cause performance regression and how much. > > Yes, I will. > In addition, are there any ramdisk test cases recommended? 😁 I think that you can start with the will-it-scale test case you used before. And you can try some workload with large number of major fault, like file read with mmap. -- Best Regards, Huang, Ying >> -- >> Best Regards, >> Huang, Ying >> >>> -- >>> Best Regards, >>> Huang, Ying >>> >>>> [1] https://github.com/antonblanchard/will-it-scale/tree/master >>>> >>>>> Regards >>>>> Yin, Fengwei >>>>> >>>>>>> Regards >>>>>>> Yin, Fengwei >>>>>>> >>>>>>>> + >>>>>>>> /* No page in the page cache at all */ >>>>>>>> count_vm_event(PGMAJFAULT); >>>>>>>> count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT);
