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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: "Huang, Ying" CC: , , , , , , , , , , References: <20231122140052.4092083-1-zhangpeng362@huawei.com> <87a5r4988r.fsf@yhuang6-desk2.ccr.corp.intel.com> From: "zhangpeng (AS)" In-Reply-To: <87a5r4988r.fsf@yhuang6-desk2.ccr.corp.intel.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.179.160] X-ClientProxiedBy: dggems704-chm.china.huawei.com (10.3.19.181) To kwepemm000020.china.huawei.com (7.193.23.93) X-CFilter-Loop: Reflected X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: EF6CBC001F X-Stat-Signature: han4khfrmprxsxek5p3sbssxa8ytyqbe X-Rspam-User: X-HE-Tag: 1700730550-850009 X-HE-Meta: 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 zrnZMEA5 +QoDIreI5cvrSr0zAd/bmhrAWbA7z7odVBQNdwA82uFnpNkLljUj114oCxWeFWmgQSy67UXtze3aC+X3pmSC4YN0LEhDL4tGFD+uDIKKRqRB6JlVgAtLgGRslgu9IP2BeFvpXhbQpWK9yIeoW/s+DFZ8e44OO4idhULIWl+nTV6XAIKuJq0HdSBACt/vYEuMoV3JzBVHUkZB3a1sf+ApLBhmc4bTjUPVR+lSipDOYRZZIG7rF2PHit9iJ8JD7de+E5bi8Lw2ZL3trd1H27FRZX4ZB38ZG7KDuZy5cX/mEd4+9FmRLFQRU+WiTmA== 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 16:36, 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 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 > Suggested-by: "Huang, Ying" > > :-) Yes! :-) >> --- >> 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; >> + } >> + > Need to deal with ptep == NULL. Although that is high impossible. If ptep == NULL, we may just need to return VM_FAULT_SIGBUS. I'll add it in the next version. Thanks! > -- > Best Regards, > Huang, Ying > >> /* No page in the page cache at all */ >> count_vm_event(PGMAJFAULT); >> count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT); -- Best Regards, Peng