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Wed, 6 Mar 2024 17:54:35 +0800 Message-ID: <3f2b71de-6d90-8b9a-cd7c-fea947a8a2ba@huawei.com> Date: Wed, 6 Mar 2024 17:54:34 +0800 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:102.0) Gecko/20100101 Thunderbird/102.9.0 Subject: Re: [PATCH v4] filemap: avoid unnecessary major faults in filemap_fault() Content-Language: en-US To: "Huang, Ying" CC: , , , , , , , , , , References: <20240306083809.1236634-1-zhangpeng362@huawei.com> <871q8n926m.fsf@yhuang6-desk2.ccr.corp.intel.com> From: "zhangpeng (AS)" In-Reply-To: <871q8n926m.fsf@yhuang6-desk2.ccr.corp.intel.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.174.179.160] X-ClientProxiedBy: dggems704-chm.china.huawei.com (10.3.19.181) To kwepemm600020.china.huawei.com (7.193.23.147) X-Rspam-User: X-Stat-Signature: 88prae6f89xwk3xkkm34er6g9skn4che X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: B60C7120006 X-HE-Tag: 1709718880-646187 X-HE-Meta: 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 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 2024/3/6 16:47, Huang, Ying wrote: > Peng Zhang writes: >> From: ZhangPeng >> >> The major fault occurred when using mlockall(MCL_CURRENT | MCL_FUTURE) >> in application, which leading to an unexpected issue[1]. >> >> This caused by temporarily cleared PTE during a read+clear/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. >> >> This issue affects our traffic analysis service. The inbound traffic is >> heavy. If a major fault occurs, the I/O schedule is triggered and the >> original I/O is suspended. Generally, the I/O schedule is 0.7 ms. If >> other applications are operating disks, the system needs to wait for >> more than 10 ms. However, the inbound traffic is heavy and the NIC buffer >> is small. As a result, packet loss occurs. But the traffic analysis service >> can't tolerate packet loss. >> >> Fix this by holding PTL and rechecking the PTE in filemap_fault() before >> triggering a major fault. We do this check only if vma is VM_LOCKED to >> reduce the performance impact in common scenarios. >> >> In our product environment, there were 7 major faults every 12 hours. >> After the patch is applied, no major fault have been triggered. >> >> Testing file 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. We do this check >> only if vma is VM_LOCKED, therefore, no performance regressions is caused >> for most common cases. >> >> The test results are as follows: >> processes processes_idle threads threads_idle >> ext4 private file write: 0.22% 0.26% 1.21% -0.15% >> ext4 private file read: 0.03% 1.00% 1.39% 0.34% >> ext4 shared file write: -0.50% -0.02% -0.14% -0.02% >> ramdisk private file write: 0.07% 0.02% 0.53% 0.04% >> ramdisk private file read: 0.01% 1.60% -0.32% -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" >> Suggested-by: David Hildenbrand >> Signed-off-by: ZhangPeng >> Signed-off-by: Kefeng Wang > LGTM, Thanks! Feel free to add > > Reviewed-by: "Huang, Ying" Thanks ! 😁 >> --- >> v3->v4: >> - Update the performance data and commit message >> - Check PTE without lock firstly per Huang, Ying >> - Update comments for recheck function per David Hildenbrand >> - Simply return 0 to make it easier to read per David Hildenbrand >> - Check !FAULT_FLAG_ORIG_PTE_VALID instead of pmd_none() >> >> v2->v3: >> - Do this check only if vma is VM_LOCKED per David Hildenbrand >> - Hold PTL and recheck the PTE >> - Place the recheck code in a new function filemap_fault_recheck_pte() >> >> v1->v2: >> - Add more test results per Huang, Ying >> - Add more comments before check PTE per Huang, Ying, David Hildenbrand >> and Yin Fengwei >> - Change pte_offset_map_nolock to pte_offset_map as the PTL won't >> be used >> >> 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 | 46 ++++++++++++++++++++++++++++++++++++++++++++++ >> 1 file changed, 46 insertions(+) >> >> diff --git a/mm/filemap.c b/mm/filemap.c >> index b4858d89f1b1..31ab455c4537 100644 >> --- a/mm/filemap.c >> +++ b/mm/filemap.c >> @@ -3181,6 +3181,48 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf, >> return fpin; >> } >> >> +static vm_fault_t filemap_fault_recheck_pte_none(struct vm_fault *vmf) >> +{ >> + struct vm_area_struct *vma = vmf->vma; >> + vm_fault_t ret = 0; >> + pte_t *ptep; >> + >> + /* >> + * We might have COW'ed a pagecache folio and might now have an mlocked >> + * anon folio mapped. The original pagecache folio is not mlocked and >> + * might have been evicted. During a read+clear/modify/write update of >> + * the PTE, such as done in do_numa_page()/change_pte_range(), we >> + * temporarily clear the PTE under PT lock and might detect it here as >> + * "none" when not holding the PT lock. >> + * >> + * Not rechecking the PTE under PT lock could result in an unexpected >> + * major fault in an mlock'ed region. Recheck only for this special >> + * scenario while holding the PT lock, to not degrade non-mlocked >> + * scenarios. Recheck the PTE without PT lock firstly, thereby reducing >> + * the number of times we hold PT lock. >> + */ >> + if (!(vma->vm_flags & VM_LOCKED)) >> + return 0; >> + >> + if (!(vmf->flags & FAULT_FLAG_ORIG_PTE_VALID)) >> + return 0; >> + >> + ptep = pte_offset_map(vmf->pmd, vmf->address); >> + if (unlikely(!ptep)) >> + return VM_FAULT_NOPAGE; >> + >> + if (unlikely(!pte_none(ptep_get_lockless(ptep)))) { >> + ret = VM_FAULT_NOPAGE; >> + } else { >> + spin_lock(vmf->ptl); >> + if (unlikely(!pte_none(ptep_get(ptep)))) >> + ret = VM_FAULT_NOPAGE; >> + spin_unlock(vmf->ptl); >> + } >> + pte_unmap(ptep); >> + return ret; >> +} >> + >> /** >> * filemap_fault - read in file data for page fault handling >> * @vmf: struct vm_fault containing details of the fault >> @@ -3236,6 +3278,10 @@ vm_fault_t filemap_fault(struct vm_fault *vmf) >> mapping_locked = true; >> } >> } else { >> + ret = filemap_fault_recheck_pte_none(vmf); >> + if (unlikely(ret)) >> + return ret; >> + >> /* No page in the page cache at all */ >> count_vm_event(PGMAJFAULT); >> count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT); -- Best Regards, Peng