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[148.163.158.5]) by mx.google.com with ESMTPS id d20si4051256eda.173.2019.04.24.00.57.23 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 24 Apr 2019 00:57:23 -0700 (PDT) Received-SPF: pass (google.com: domain of ldufour@linux.ibm.com designates 148.163.158.5 as permitted sender) client-ip=148.163.158.5; Authentication-Results: mx.google.com; spf=pass (google.com: domain of ldufour@linux.ibm.com designates 148.163.158.5 as permitted sender) smtp.mailfrom=ldufour@linux.ibm.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=ibm.com Received: from pps.filterd (m0098419.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.27/8.16.0.27) with SMTP id x3O7nm5a137639 for ; Wed, 24 Apr 2019 03:57:22 -0400 Received: from e06smtp03.uk.ibm.com (e06smtp03.uk.ibm.com [195.75.94.99]) by mx0b-001b2d01.pphosted.com with ESMTP id 2s2k57a789-1 (version=TLSv1.2 cipher=AES256-GCM-SHA384 bits=256 verify=NOT) for ; Wed, 24 Apr 2019 03:57:21 -0400 Received: from localhost by e06smtp03.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! 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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Wed, 24 Apr 2019 08:57:10 +0100 Received: from d06av23.portsmouth.uk.ibm.com (d06av23.portsmouth.uk.ibm.com [9.149.105.59]) by b06cxnps3075.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x3O7v8vK29819118 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Wed, 24 Apr 2019 07:57:08 GMT Received: from d06av23.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 7DA4CA4051; Wed, 24 Apr 2019 07:57:08 +0000 (GMT) Received: from d06av23.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id CFE5FA4040; Wed, 24 Apr 2019 07:57:05 +0000 (GMT) Received: from [9.145.184.124] (unknown [9.145.184.124]) by d06av23.portsmouth.uk.ibm.com (Postfix) with ESMTP; Wed, 24 Apr 2019 07:57:05 +0000 (GMT) Subject: Re: [PATCH v12 21/31] mm: Introduce find_vma_rcu() To: Peter Zijlstra Cc: akpm@linux-foundation.org, mhocko@kernel.org, kirill@shutemov.name, ak@linux.intel.com, dave@stgolabs.net, jack@suse.cz, Matthew Wilcox , aneesh.kumar@linux.ibm.com, benh@kernel.crashing.org, mpe@ellerman.id.au, paulus@samba.org, Thomas Gleixner , Ingo Molnar , hpa@zytor.com, Will Deacon , Sergey Senozhatsky , sergey.senozhatsky.work@gmail.com, Andrea Arcangeli , Alexei Starovoitov , kemi.wang@intel.com, Daniel Jordan , David Rientjes , Jerome Glisse , Ganesh Mahendran , Minchan Kim , Punit Agrawal , vinayak menon , Yang Shi , zhong jiang , Haiyan Song , Balbir Singh , sj38.park@gmail.com, Michel Lespinasse , Mike Rapoport , linux-kernel@vger.kernel.org, linux-mm@kvack.org, haren@linux.vnet.ibm.com, npiggin@gmail.com, paulmck@linux.vnet.ibm.com, Tim Chen , linuxppc-dev@lists.ozlabs.org, x86@kernel.org References: <20190416134522.17540-1-ldufour@linux.ibm.com> <20190416134522.17540-22-ldufour@linux.ibm.com> <20190423092710.GI11158@hirez.programming.kicks-ass.net> From: Laurent Dufour Date: Wed, 24 Apr 2019 09:57:05 +0200 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <20190423092710.GI11158@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 x-cbid: 19042407-0012-0000-0000-00000312C636 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19042407-0013-0000-0000-0000214B1A4D Message-Id: X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-04-24_05:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=907 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1904240068 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: Le 23/04/2019 à 11:27, Peter Zijlstra a écrit : > On Tue, Apr 16, 2019 at 03:45:12PM +0200, Laurent Dufour wrote: >> This allows to search for a VMA structure without holding the mmap_sem. >> >> The search is repeated while the mm seqlock is changing and until we found >> a valid VMA. >> >> While under the RCU protection, a reference is taken on the VMA, so the >> caller must call put_vma() once it not more need the VMA structure. >> >> At the time a VMA is inserted in the MM RB tree, in vma_rb_insert(), a >> reference is taken to the VMA by calling get_vma(). >> >> When removing a VMA from the MM RB tree, the VMA is not release immediately >> but at the end of the RCU grace period through vm_rcu_put(). This ensures >> that the VMA remains allocated until the end the RCU grace period. >> >> Since the vm_file pointer, if valid, is released in put_vma(), there is no >> guarantee that the file pointer will be valid on the returned VMA. > > What I'm missing here, and in the previous patch introducing the > refcount (also see refcount_t), is _why_ we need the refcount thing at > all. The need for the VMA's refcount is to ensure that the VMA will remain until the end of the SPF handler. This is a consequence of the use of RCU instead of SRCU to protect the RB tree. I was not aware of the refcount_t type, it would be better here to avoid wrapping. > My original plan was to use SRCU, which at the time was not complete > enough so I abused/hacked preemptible RCU, but that is no longer the > case, SRCU has all the required bits and pieces. When I did test using SRCU it was involving a lot a scheduling to run the SRCU callback mechanism. In some workload the impact on the perfomance was significant [1]. I can't see this overhead using RCU. > > Also; the initial motivation was prefaulting large VMAs and the > contention on mmap was killing things; but similarly, the contention on > the refcount (I did try that) killed things just the same. Doing prefaulting should be doable, I'll try to think further about that. Regarding the refcount, I should I missed something, this is an atomic counter, so there should not be contention on it but cache exclusivity, not ideal I agree but I can't see what else to use here. > So I'm really sad to see the refcount return; and without any apparent > justification. I'm not opposed to use another mechanism here, but SRCU didn't show good performance with some workload, and I can't see how to use RCU without a reference counter here. So please, advise. Thanks, Laurent. [1] https://lore.kernel.org/linux-mm/7ca80231-fe02-a3a7-84bc-ce81690ea051@intel.com/