From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754705AbeDCALq (ORCPT ); Mon, 2 Apr 2018 20:11:46 -0400 Received: from mail-pl0-f68.google.com ([209.85.160.68]:41742 "EHLO mail-pl0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754550AbeDCALo (ORCPT ); Mon, 2 Apr 2018 20:11:44 -0400 X-Google-Smtp-Source: AIpwx49zHzYrALZT47sMYF6wGENGdZXteMQJoZSb/7BEWiYLkc4OPmy5uck1VL+RDnx5F5R+QS/0Sw== Date: Mon, 2 Apr 2018 17:11:42 -0700 (PDT) From: David Rientjes X-X-Sender: rientjes@chino.kir.corp.google.com To: Laurent Dufour cc: paulmck@linux.vnet.ibm.com, peterz@infradead.org, akpm@linux-foundation.org, kirill@shutemov.name, ak@linux.intel.com, mhocko@kernel.org, dave@stgolabs.net, jack@suse.cz, Matthew Wilcox , benh@kernel.crashing.org, mpe@ellerman.id.au, paulus@samba.org, Thomas Gleixner , Ingo Molnar , hpa@zytor.com, Will Deacon , Sergey Senozhatsky , Andrea Arcangeli , Alexei Starovoitov , kemi.wang@intel.com, sergey.senozhatsky.work@gmail.com, Daniel Jordan , linux-kernel@vger.kernel.org, linux-mm@kvack.org, haren@linux.vnet.ibm.com, khandual@linux.vnet.ibm.com, npiggin@gmail.com, bsingharora@gmail.com, Tim Chen , linuxppc-dev@lists.ozlabs.org, x86@kernel.org Subject: Re: [PATCH v9 17/24] mm: Protect mm_rb tree with a rwlock In-Reply-To: <1520963994-28477-18-git-send-email-ldufour@linux.vnet.ibm.com> Message-ID: References: <1520963994-28477-1-git-send-email-ldufour@linux.vnet.ibm.com> <1520963994-28477-18-git-send-email-ldufour@linux.vnet.ibm.com> User-Agent: Alpine 2.20 (DEB 67 2015-01-07) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 13 Mar 2018, Laurent Dufour wrote: > This change is inspired by the Peter's proposal patch [1] which was > protecting the VMA using SRCU. Unfortunately, SRCU is not scaling well in > that particular case, and it is introducing major performance degradation > due to excessive scheduling operations. > > To allow access to the mm_rb tree without grabbing the mmap_sem, this patch > is protecting it access using a rwlock. As the mm_rb tree is a O(log n) > search it is safe to protect it using such a lock. The VMA cache is not > protected by the new rwlock and it should not be used without holding the > mmap_sem. > > To allow the picked VMA structure to be used once the rwlock is released, a > use count is added to the VMA structure. When the VMA is allocated it is > set to 1. Each time the VMA is picked with the rwlock held its use count > is incremented. Each time the VMA is released it is decremented. When the > use count hits zero, this means that the VMA is no more used and should be > freed. > > This patch is preparing for 2 kind of VMA access : > - as usual, under the control of the mmap_sem, > - without holding the mmap_sem for the speculative page fault handler. > > Access done under the control the mmap_sem doesn't require to grab the > rwlock to protect read access to the mm_rb tree, but access in write must > be done under the protection of the rwlock too. This affects inserting and > removing of elements in the RB tree. > > The patch is introducing 2 new functions: > - vma_get() to find a VMA based on an address by holding the new rwlock. > - vma_put() to release the VMA when its no more used. > These services are designed to be used when access are made to the RB tree > without holding the mmap_sem. > > When a VMA is removed from the RB tree, its vma->vm_rb field is cleared and > we rely on the WMB done when releasing the rwlock to serialize the write > with the RMB done in a later patch to check for the VMA's validity. > > When free_vma is called, the file associated with the VMA is closed > immediately, but the policy and the file structure remained in used until > the VMA's use count reach 0, which may happens later when exiting an > in progress speculative page fault. > > [1] https://patchwork.kernel.org/patch/5108281/ > > Cc: Peter Zijlstra (Intel) > Cc: Matthew Wilcox > Signed-off-by: Laurent Dufour Can __free_vma() be generalized for mm/nommu.c's delete_vma() and do_mmap()?