From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S968429AbdIZJXG (ORCPT ); Tue, 26 Sep 2017 05:23:06 -0400 Received: from szxga05-in.huawei.com ([45.249.212.191]:7022 "EHLO szxga05-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S966927AbdIZJXC (ORCPT ); Tue, 26 Sep 2017 05:23:02 -0400 Message-ID: <59CA1C6E.4010501@huawei.com> Date: Tue, 26 Sep 2017 17:22:54 +0800 From: Xishi Qiu User-Agent: Mozilla/5.0 (Windows NT 6.1; rv:12.0) Gecko/20120428 Thunderbird/12.0.1 MIME-Version: 1.0 To: Michal Hocko CC: Joonsoo Kim , Vlastimil Babka , Mel Gorman , Linux MM , LKML , zhong jiang , yeyunfeng , , "Zhoukang (A)" Subject: Re: [RFC] a question about mlockall() and mprotect() References: <59CA0847.8000508@huawei.com> <20170926081716.xo375arjoyu5ytcb@dhcp22.suse.cz> <59CA125C.8000801@huawei.com> <20170926090255.jmocezs6s3lpd6p4@dhcp22.suse.cz> <59CA1A57.5000905@huawei.com> In-Reply-To: <59CA1A57.5000905@huawei.com> Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit X-Originating-IP: [10.177.25.179] X-CFilter-Loop: Reflected X-Mirapoint-Virus-RAPID-Raw: score=unknown(0), refid=str=0001.0A090204.59CA1C75.0024,ss=1,re=0.000,recu=0.000,reip=0.000,cl=1,cld=1,fgs=0, ip=0.0.0.0, so=2014-11-16 11:51:01, dmn=2013-03-21 17:37:32 X-Mirapoint-Loop-Id: 680568639d747f2a0e3590340e04255b Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2017/9/26 17:13, Xishi Qiu wrote: > On 2017/9/26 17:02, Michal Hocko wrote: > >> On Tue 26-09-17 16:39:56, Xishi Qiu wrote: >>> On 2017/9/26 16:17, Michal Hocko wrote: >>> >>>> On Tue 26-09-17 15:56:55, Xishi Qiu wrote: >>>>> When we call mlockall(), we will add VM_LOCKED to the vma, >>>>> if the vma prot is ---p, >>>> >>>> not sure what you mean here. apply_mlockall_flags will set the flag on >>>> all vmas except for special mappings (mlock_fixup). This phase will >>>> cause that memory reclaim will not free already mapped pages in those >>>> vmas (see page_check_references and the lazy mlock pages move to >>>> unevictable LRUs). >>>> >>>>> then mm_populate -> get_user_pages will not alloc memory. >>>> >>>> mm_populate all the vmas with pages. Well there are certainly some >>>> constrains - e.g. memory cgroup hard limit might be hit and so the >>>> faulting might fail. >>>> >>>>> I find it said "ignore errors" in mm_populate() >>>>> static inline void mm_populate(unsigned long addr, unsigned long len) >>>>> { >>>>> /* Ignore errors */ >>>>> (void) __mm_populate(addr, len, 1); >>>>> } >>>> >>>> But we do not report the failure because any failure past >>>> apply_mlockall_flags would be tricky to handle. We have already dropped >>>> the mmap_sem lock so some other address space operations could have >>>> interfered. >>>> >>>>> And later we call mprotect() to change the prot, then it is >>>>> still not alloc memory for the mlocked vma. >>>>> >>>>> My question is that, shall we alloc memory if the prot changed, >>>>> and who(kernel, glibc, user) should alloc the memory? >>>> >>>> I do not understand your question but if you are asking how to get pages >>>> to map your vmas then touching that area will fault the memory in. >>> >>> Hi Michal, >>> >>> syscall mlockall() will first apply the VM_LOCKED to the vma, then >>> call mm_populate() to map the vmas. >>> >>> mm_populate >>> populate_vma_page_range >>> __get_user_pages >>> check_vma_flags >>> And the above path maybe return -EFAULT in some case, right? >>> >>> If we call mprotect() to change the prot of vma, just let >>> check_vma_flags() return 0, then we will get the mlocked pages >>> in following page-fault, right? >> >> Any future page fault to the existing vma will result in the mlocked >> page. That is what VM_LOCKED guarantess. >> >>> My question is that, shall we map the vmas immediately when >>> the prot changed? If we should map it immediately, who(kernel, glibc, user) >>> do this step? >> >> This is still very fuzzy. What are you actually trying to achieve? > > I don't expect page fault any more after mlock. > Our apps is some thing like RT, and page-fault maybe cause a lot of time, e.g. lock, mem reclaim ..., so I use mlock and don't want page fault any more. Thanks, Xishi Qiu > > . >