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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4011CC6FA82 for ; Tue, 13 Sep 2022 02:15:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229888AbiIMCPD (ORCPT ); Mon, 12 Sep 2022 22:15:03 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52562 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229629AbiIMCPB (ORCPT ); Mon, 12 Sep 2022 22:15:01 -0400 Received: from szxga08-in.huawei.com (szxga08-in.huawei.com [45.249.212.255]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id B7B851CFED for ; Mon, 12 Sep 2022 19:14:58 -0700 (PDT) Received: from canpemm500002.china.huawei.com (unknown [172.30.72.56]) by szxga08-in.huawei.com (SkyGuard) with ESMTP id 4MRRlS6L3Bz14QZQ; Tue, 13 Sep 2022 10:11:00 +0800 (CST) Received: from [10.174.177.76] (10.174.177.76) by canpemm500002.china.huawei.com (7.192.104.244) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2375.31; Tue, 13 Sep 2022 10:14:55 +0800 Subject: Re: [PATCH 8/8] hugetlb: use new vma_lock for pmd sharing synchronization To: Mike Kravetz CC: Muchun Song , David Hildenbrand , Michal Hocko , Peter Xu , Naoya Horiguchi , "Aneesh Kumar K . V" , Andrea Arcangeli , "Kirill A . Shutemov" , Davidlohr Bueso , Prakash Sangappa , James Houghton , Mina Almasry , Pasha Tatashin , Axel Rasmussen , Ray Fucillo , Andrew Morton , , References: <20220824175757.20590-1-mike.kravetz@oracle.com> <20220824175757.20590-9-mike.kravetz@oracle.com> <08edc08e-08ab-0706-3c8d-804080f37bd7@huawei.com> <1baff74d-d38f-6139-2548-19c0c8f87649@huawei.com> From: Miaohe Lin Message-ID: Date: Tue, 13 Sep 2022 10:14:55 +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: [10.174.177.76] X-ClientProxiedBy: dggems701-chm.china.huawei.com (10.3.19.178) To canpemm500002.china.huawei.com (7.192.104.244) X-CFilter-Loop: Reflected Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2022/9/13 7:02, Mike Kravetz wrote: > On 09/05/22 11:08, Miaohe Lin wrote: >> On 2022/9/3 7:07, Mike Kravetz wrote: >>> On 08/30/22 10:02, Miaohe Lin wrote: >>>> On 2022/8/25 1:57, Mike Kravetz wrote: >>>>> The new hugetlb vma lock (rw semaphore) is used to address this race: >>>>> >>>>> Faulting thread Unsharing thread >>>>> ... ... >>>>> ptep = huge_pte_offset() >>>>> or >>>>> ptep = huge_pte_alloc() >>>>> ... >>>>> i_mmap_lock_write >>>>> lock page table >>>>> ptep invalid <------------------------ huge_pmd_unshare() >>>>> Could be in a previously unlock_page_table >>>>> sharing process or worse i_mmap_unlock_write >>>>> ... >>>>> >>>>> The vma_lock is used as follows: >>>>> - During fault processing. the lock is acquired in read mode before >>>>> doing a page table lock and allocation (huge_pte_alloc). The lock is >>>>> held until code is finished with the page table entry (ptep). >>>>> - The lock must be held in write mode whenever huge_pmd_unshare is >>>>> called. >>>>> >>>>> Lock ordering issues come into play when unmapping a page from all >>>>> vmas mapping the page. The i_mmap_rwsem must be held to search for the >>>>> vmas, and the vma lock must be held before calling unmap which will >>>>> call huge_pmd_unshare. This is done today in: >>>>> - try_to_migrate_one and try_to_unmap_ for page migration and memory >>>>> error handling. In these routines we 'try' to obtain the vma lock and >>>>> fail to unmap if unsuccessful. Calling routines already deal with the >>>>> failure of unmapping. >>>>> - hugetlb_vmdelete_list for truncation and hole punch. This routine >>>>> also tries to acquire the vma lock. If it fails, it skips the >>>>> unmapping. However, we can not have file truncation or hole punch >>>>> fail because of contention. After hugetlb_vmdelete_list, truncation >>>>> and hole punch call remove_inode_hugepages. remove_inode_hugepages >>>>> check for mapped pages and call hugetlb_unmap_file_page to unmap them. >>>>> hugetlb_unmap_file_page is designed to drop locks and reacquire in the >>>>> correct order to guarantee unmap success. >>>>> >>>>> Signed-off-by: Mike Kravetz >>>>> --- >>>>> fs/hugetlbfs/inode.c | 46 +++++++++++++++++++ >>>>> mm/hugetlb.c | 102 +++++++++++++++++++++++++++++++++++++++---- >>>>> mm/memory.c | 2 + >>>>> mm/rmap.c | 100 +++++++++++++++++++++++++++--------------- >>>>> mm/userfaultfd.c | 9 +++- >>>>> 5 files changed, 214 insertions(+), 45 deletions(-) >>>>> >>>>> diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c >>>>> index b93d131b0cb5..52d9b390389b 100644 >>>>> --- a/fs/hugetlbfs/inode.c >>>>> +++ b/fs/hugetlbfs/inode.c >>>>> @@ -434,6 +434,8 @@ static void hugetlb_unmap_file_folio(struct hstate *h, >>>>> struct folio *folio, pgoff_t index) >>>>> { >>>>> struct rb_root_cached *root = &mapping->i_mmap; >>>>> + unsigned long skipped_vm_start; >>>>> + struct mm_struct *skipped_mm; >>>>> struct page *page = &folio->page; >>>>> struct vm_area_struct *vma; >>>>> unsigned long v_start; >>>>> @@ -444,6 +446,8 @@ static void hugetlb_unmap_file_folio(struct hstate *h, >>>>> end = ((index + 1) * pages_per_huge_page(h)); >>>>> >>>>> i_mmap_lock_write(mapping); >>>>> +retry: >>>>> + skipped_mm = NULL; >>>>> >>>>> vma_interval_tree_foreach(vma, root, start, end - 1) { >>>>> v_start = vma_offset_start(vma, start); >>>>> @@ -452,11 +456,49 @@ static void hugetlb_unmap_file_folio(struct hstate *h, >>>>> if (!hugetlb_vma_maps_page(vma, vma->vm_start + v_start, page)) >>>>> continue; >>>>> >>>>> + if (!hugetlb_vma_trylock_write(vma)) { >>>>> + /* >>>>> + * If we can not get vma lock, we need to drop >>>>> + * immap_sema and take locks in order. >>>>> + */ >>>>> + skipped_vm_start = vma->vm_start; >>>>> + skipped_mm = vma->vm_mm; >>>>> + /* grab mm-struct as we will be dropping i_mmap_sema */ >>>>> + mmgrab(skipped_mm); >>>>> + break; >>>>> + } >>>>> + >>>>> unmap_hugepage_range(vma, vma->vm_start + v_start, v_end, >>>>> NULL, ZAP_FLAG_DROP_MARKER); >>>>> + hugetlb_vma_unlock_write(vma); >>>>> } >>>>> >>>>> i_mmap_unlock_write(mapping); >>>>> + >>>>> + if (skipped_mm) { >>>>> + mmap_read_lock(skipped_mm); >>>>> + vma = find_vma(skipped_mm, skipped_vm_start); >>>>> + if (!vma || !is_vm_hugetlb_page(vma) || >>>>> + vma->vm_file->f_mapping != mapping || >>>>> + vma->vm_start != skipped_vm_start) { >>>> >>>> i_mmap_lock_write(mapping) is missing here? Retry logic will do i_mmap_unlock_write(mapping) anyway. >>>> >>> >>> Yes, that is missing. I will add here. >>> >>>>> + mmap_read_unlock(skipped_mm); >>>>> + mmdrop(skipped_mm); >>>>> + goto retry; >>>>> + } >>>>> + >>>> >>>> IMHO, above check is not enough. Think about the below scene: >>>> >>>> CPU 1 CPU 2 >>>> hugetlb_unmap_file_folio exit_mmap >>>> mmap_read_lock(skipped_mm); mmap_read_lock(mm); >>>> check vma is wanted. >>>> unmap_vmas >>>> mmap_read_unlock(skipped_mm); mmap_read_unlock >>>> mmap_write_lock(mm); >>>> free_pgtables >>>> remove_vma >>>> hugetlb_vma_lock_free >>>> vma, hugetlb_vma_lock is still *used after free* >>>> mmap_write_unlock(mm); >>>> So we should check mm->mm_users == 0 to fix the above issue. Or am I miss something? >>> >>> In the retry case, we are OK because go back and look up the vma again. Right? >>> >>> After taking mmap_read_lock, vma can not go away until we mmap_read_unlock. >>> Before that, we do the following: >>> >>>>> + hugetlb_vma_lock_write(vma); >>>>> + i_mmap_lock_write(mapping); >>> >>> IIUC, vma can not go away while we hold i_mmap_lock_write. So, after this we >> >> I think you're right. free_pgtables() can't complete its work as unlink_file_vma() will be >> blocked on i_mmap_rwsem of mapping. Sorry for reporting such nonexistent race. >> >>> can. >>> >>>>> + mmap_read_unlock(skipped_mm); >>>>> + mmdrop(skipped_mm); >>> >>> We continue to hold i_mmap_lock_write as we goto retry. >>> >>> I could be missing something as well. This was how I intended to keep >>> vma valid while dropping and acquiring locks. >> >> Thanks for your clarifying. >> > > Well, that was all correct 'in theory' but not in practice. I did not take > into account the inode lock that is taken at the beginning of truncate (or > hole punch). In other code paths, we take inode lock after mmap_lock. So, > taking mmap_lock here is not allowed. Considering the Lock ordering in mm/filemap.c: * ->i_rwsem * ->invalidate_lock (acquired by fs in truncate path) * ->i_mmap_rwsem (truncate->unmap_mapping_range) * ->i_rwsem (generic_perform_write) * ->mmap_lock (fault_in_readable->do_page_fault) It seems inode_lock is taken before the mmap_lock? Thanks, Miaohe Lin > > I came up with another way to make this work. As discussed above, we need to > drop the i_mmap lock before acquiring the vma_lock. However, once we drop > i_mmap, the vma could go away. My solution is to make the 'vma_lock' be a > ref counted structure that can live on after the vma is freed. Therefore, > this code can take a reference while under i_mmap then drop i_mmap and wait > on the vma_lock. Of course, once it acquires the vma_lock it needs to check > and make sure the vma still exists. It may sound complicated, but I think > it is a bit simpler than the code here. A new series will be out soon. >