From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 89CEBC43460 for ; Thu, 8 Apr 2021 03:26:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 65D7561186 for ; Thu, 8 Apr 2021 03:26:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229809AbhDHD0Y (ORCPT ); Wed, 7 Apr 2021 23:26:24 -0400 Received: from szxga04-in.huawei.com ([45.249.212.190]:15169 "EHLO szxga04-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229505AbhDHD0U (ORCPT ); Wed, 7 Apr 2021 23:26:20 -0400 Received: from DGGEMS404-HUB.china.huawei.com (unknown [172.30.72.58]) by szxga04-in.huawei.com (SkyGuard) with ESMTP id 4FG66K3FkQzpWKr; Thu, 8 Apr 2021 11:23:21 +0800 (CST) Received: from [10.174.179.9] (10.174.179.9) by DGGEMS404-HUB.china.huawei.com (10.3.19.204) with Microsoft SMTP Server id 14.3.498.0; Thu, 8 Apr 2021 11:26:05 +0800 Subject: Re: [PATCH 3/4] mm/hugeltb: fix potential wrong gbl_reserve value for hugetlb_acct_memory() From: Miaohe Lin To: Mike Kravetz , CC: , , References: <20210402093249.25137-1-linmiaohe@huawei.com> <20210402093249.25137-4-linmiaohe@huawei.com> <20afccd5-2bc4-9db9-695e-dd6175b0b42b@oracle.com> <1311fcfe-bc5f-e878-3912-ca9a9e0eed90@huawei.com> Message-ID: <0ebaa062-80e8-b380-c02e-7eb72e67f973@huawei.com> Date: Thu, 8 Apr 2021 11:26:05 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset="utf-8" Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.179.9] X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2021/4/8 11:24, Miaohe Lin wrote: > On 2021/4/8 4:53, Mike Kravetz wrote: >> On 4/7/21 12:24 AM, Miaohe Lin wrote: >>> Hi: >>> On 2021/4/7 10:49, Mike Kravetz wrote: >>>> On 4/2/21 2:32 AM, Miaohe Lin wrote: >>>>> The resv_map could be NULL since this routine can be called in the evict >>>>> inode path for all hugetlbfs inodes. So we could have chg = 0 and this >>>>> would result in a negative value when chg - freed. This is unexpected for >>>>> hugepage_subpool_put_pages() and hugetlb_acct_memory(). >>>> >>>> I am not sure if this is possible. >>>> >>>> It is true that resv_map could be NULL. However, I believe resv map >>>> can only be NULL for inodes that are not regular or link inodes. This >>>> is the inode creation code in hugetlbfs_get_inode(). >>>> >>>> /* >>>> * Reserve maps are only needed for inodes that can have associated >>>> * page allocations. >>>> */ >>>> if (S_ISREG(mode) || S_ISLNK(mode)) { >>>> resv_map = resv_map_alloc(); >>>> if (!resv_map) >>>> return NULL; >>>> } >>>> >>> >>> Agree. >>> >>>> If resv_map is NULL, then no hugetlb pages can be allocated/associated >>>> with the file. As a result, remove_inode_hugepages will never find any >>>> huge pages associated with the inode and the passed value 'freed' will >>>> always be zero. >>>> >>> >>> But I am confused now. AFAICS, remove_inode_hugepages() searches the address_space of >>> the inode to remove the hugepages while does not care if inode has associated resv_map. >>> How does it prevent hugetlb pages from being allocated/associated with the file if >>> resv_map is NULL? Could you please explain this more? >>> >> >> Recall that there are only two ways to get huge pages associated with >> a hugetlbfs file: fallocate and mmap/write fault. Directly writing to >> hugetlbfs files is not supported. >> >> If you take a closer look at hugetlbfs_get_inode, it has that code to >> allocate the resv map mentioned above as well as the following: >> >> switch (mode & S_IFMT) { >> default: >> init_special_inode(inode, mode, dev); >> break; >> case S_IFREG: >> inode->i_op = &hugetlbfs_inode_operations; >> inode->i_fop = &hugetlbfs_file_operations; >> break; >> case S_IFDIR: >> inode->i_op = &hugetlbfs_dir_inode_operations; >> inode->i_fop = &simple_dir_operations; >> >> /* directory inodes start off with i_nlink == 2 (for "." entry) */ >> inc_nlink(inode); >> break; >> case S_IFLNK: >> inode->i_op = &page_symlink_inode_operations; >> inode_nohighmem(inode); >> break; >> } >> >> Notice that only S_IFREG inodes will have i_fop == &hugetlbfs_file_operations. >> hugetlbfs_file_operations contain the hugetlbfs specific mmap and fallocate >> routines. Hence, only files with S_IFREG inodes can potentially have >> associated huge pages. S_IFLNK inodes can as well via file linking. >> >> If an inode is not S_ISREG(mode) || S_ISLNK(mode), then it will not have >> a resv_map. In addition, it will not have hugetlbfs_file_operations and >> can not have associated huge pages. >> > > Many many thanks for detailed and patient explanation! :) I think I have got the idea! > >> I looked at this closely when adding commits >> 58b6e5e8f1ad hugetlbfs: fix memory leak for resv_map >> f27a5136f70a hugetlbfs: always use address space in inode for resv_map pointer >> >> I may not be remembering all of the details correctly. Commit f27a5136f70a >> added the comment that resv_map could be NULL to hugetlb_unreserve_pages. >> > > Since we must have freed == 0 while chg == 0. Should we make this assumption explict > by something like below? > > WARN_ON(chg < freed); > Or just a comment to avoid confusion ? > Thanks again! >