"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. -- 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);
