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Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.9.0 Subject: Re: [RFC PATCH] mm: filemap: avoid unnecessary major faults in filemap_fault() Content-Language: en-US To: Yin Fengwei , , CC: , , , , , , , , References: <20231122140052.4092083-1-zhangpeng362@huawei.com> <801bd0c9-7d0c-4231-93e5-7532e8231756@intel.com> <48235d73-3dc6-263d-7822-6d479b753d46@huawei.com> <1f29e229-d07a-41e2-97da-b6720bd5f2d7@intel.com> From: "zhangpeng (AS)" In-Reply-To: <1f29e229-d07a-41e2-97da-b6720bd5f2d7@intel.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.174.179.160] X-ClientProxiedBy: dggems701-chm.china.huawei.com (10.3.19.178) To kwepemm000020.china.huawei.com (7.193.23.93) X-CFilter-Loop: Reflected X-Rspamd-Queue-Id: 3E038100010 X-Rspam-User: X-Rspamd-Server: rspam05 X-Stat-Signature: dq86mb4ujj8q4w471rpnos6z61bgonau X-HE-Tag: 1700730547-494048 X-HE-Meta: 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 wEj8eVHn YeuNA38BVgSU9ywI3puTRnUD7bOmu4I8VY5Az/0+hPDtjKMc/mCg4CISoNe7gqmj6sRG+2SBPKJNztJFea5NbzZ/cvOPSlDTAGUpeqGS1InPJXWgIGzFaZqQwYrXkmW9a+O0TrxRxVeWXOWK1y69wuFd3AYQfVPQU+QnWJPKwz5RoIxzv5x4rqmBpoLPvtMiV4GrkOHTmQcOneSlRVcvNs7TDbFEhCBbNQGBraZR6lpvDEJpFMq+4uVlwdsPFaGIZ7OUDsaChUjqKlVuIuoCzn2uHqga+KfdYq+2hhP31U7zXmatuPLfuA0NpBg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2023/11/23 16:29, Yin Fengwei wrote: > On 11/23/23 15:57, zhangpeng (AS) wrote: >> 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 >>>>>> >>>>>> 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@huawei.com/ >>>>>> >>>>>> Signed-off-by: ZhangPeng >>>>>> Signed-off-by: Kefeng Wang >>>>>> --- >>>>>>    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. > Thanks for the testing. > >>>> 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. >> >> [1] https://github.com/antonblanchard/will-it-scale/tree/master > Maybe include this performance related information to commit message? Sure, I'll add it in the next version. > For the code change, looks good to me. > > Reviewed-by: Yin Fengwei Thanks! > > Regards > Yin, Fengwei > >>> 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); -- Best Regards, Peng