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Thu, 07 Nov 2019 21:56:48 +0000 Received: from pps.filterd (aserp3020.oracle.com [127.0.0.1]) by aserp3020.oracle.com (8.16.0.27/8.16.0.27) with SMTP id xA7LsG4p196563; Thu, 7 Nov 2019 21:56:47 GMT Received: from userv0122.oracle.com (userv0122.oracle.com [156.151.31.75]) by aserp3020.oracle.com with ESMTP id 2w4k2x6egg-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Thu, 07 Nov 2019 21:56:47 +0000 Received: from abhmp0011.oracle.com (abhmp0011.oracle.com [141.146.116.17]) by userv0122.oracle.com (8.14.4/8.14.4) with ESMTP id xA7Luj0B016191; Thu, 7 Nov 2019 21:56:45 GMT Received: from [192.168.1.206] (/71.63.128.209) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Thu, 07 Nov 2019 13:56:45 -0800 Subject: Re: [PATCH] hugetlbfs: Take read_lock on i_mmap for PMD sharing From: Mike Kravetz To: Matthew Wilcox , Waiman Long Cc: Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Davidlohr Bueso , Peter Zijlstra , Ingo Molnar , Will Deacon References: <20191107190628.22667-1-longman@redhat.com> <20191107195441.GF11823@bombadil.infradead.org> Message-ID: <136eb24b-049e-9ebf-598d-1292d61d49fd@oracle.com> Date: Thu, 7 Nov 2019 13:56:43 -0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9434 signatures=668685 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=0 malwarescore=0 phishscore=0 bulkscore=0 spamscore=0 mlxscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1910280000 definitions=main-1911070202 X-Proofpoint-Virus-Version: vendor=nai engine=6000 definitions=9434 signatures=668685 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1910280000 definitions=main-1911070202 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: On 11/7/19 1:49 PM, Mike Kravetz wrote: > On 11/7/19 11:54 AM, Matthew Wilcox wrote: >> Are there other current users of the write lock that could use a read lock? >> At first blush, it would seem that unmap_ref_private() also only needs >> a read lock on the i_mmap tree. I don't think hugetlb_change_protection() >> needs the write lock either. Nor retract_page_tables(). Sorry, I missed retract_page_tables which is not part of hugetlb code. The comments below do not apply to retract_page_tables. Someone would need to take a closer look to see if that really needs write mode. -- Mike Kravetz > > I believe that the semaphore still needs to be held in write mode while > calling huge_pmd_unshare (as is done in the call sites above). Why? > There is this check for sharing in huge_pmd_unshare, > > if (page_count(virt_to_page(ptep)) == 1) > return 0; // implies no sharing > > Note that huge_pmd_share now increments the page count with the semaphore > held just in read mode. It is OK to do increments in parallel without > synchronization. However, we don't want anyone else changing the count > while that check in huge_pmd_unshare is happening. Hence, the need for > taking the semaphore in write mode. >