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=-19.1 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1,USER_IN_DEF_DKIM_WL autolearn=unavailable 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 D4198C433E0 for ; Wed, 29 Jul 2020 19:21:55 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 7BC3B2068F for ; Wed, 29 Jul 2020 19:21:55 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="n8so8KYJ" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 7BC3B2068F Authentication-Results: mail.kernel.org; dmarc=fail (p=reject dis=none) header.from=google.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id E09676B0002; Wed, 29 Jul 2020 15:21:54 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id DB9076B0003; Wed, 29 Jul 2020 15:21:54 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id CA7176B0006; Wed, 29 Jul 2020 15:21:54 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0225.hostedemail.com [216.40.44.225]) by kanga.kvack.org (Postfix) with ESMTP id B596C6B0002 for ; Wed, 29 Jul 2020 15:21:54 -0400 (EDT) Received: from smtpin11.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay03.hostedemail.com (Postfix) with ESMTP id 6A24E8248D51 for ; Wed, 29 Jul 2020 19:21:54 +0000 (UTC) X-FDA: 77092083348.11.stew92_0d05eaa26f75 Received: from filter.hostedemail.com (10.5.16.251.rfc1918.com [10.5.16.251]) by smtpin11.hostedemail.com (Postfix) with ESMTP id 7269D180F8B86 for ; Wed, 29 Jul 2020 19:21:19 +0000 (UTC) X-HE-Tag: stew92_0d05eaa26f75 X-Filterd-Recvd-Size: 7537 Received: from mail-qk1-f194.google.com (mail-qk1-f194.google.com [209.85.222.194]) by imf48.hostedemail.com (Postfix) with ESMTP for ; Wed, 29 Jul 2020 19:21:18 +0000 (UTC) Received: by mail-qk1-f194.google.com with SMTP id h7so23339002qkk.7 for ; Wed, 29 Jul 2020 12:21:18 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=date:from:to:cc:subject:in-reply-to:message-id:references :user-agent:mime-version; bh=xf8y6+QE7u4z9tDYoWpKfnyILr6WJGhsLpbcNKIkelU=; b=n8so8KYJL7JTmU4FRh9v0OQdg1omhOYvRSE92qmDvKlITdi/LJg3OMvlrRg7KCISxw GDiEDm6vbZDm4z7zCnQ9gOZen6GBUgOVlhajnAsr/bMFrhTaaNfLHX6ukTqQc3u9ykaz vpfHcgO1dQPjfiB4BBf0JmXcZAONXwPrRp/fHP/7yMRdFvHzaxud620vIIfCJkM31y/u L9Vuw+e+5PV9CF1QdpRziQUdRphSv2XcZkdlItKQQib8zHIYmj31ZVVNPDcL7HUx1bYa D4ZPuZTk8pwt0T18C2F5ftoc4JBv5FbT7S2naq/vJtjg8z0HqS+SFgPElReYcxMN1Ewn bkOQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:in-reply-to:message-id :references:user-agent:mime-version; bh=xf8y6+QE7u4z9tDYoWpKfnyILr6WJGhsLpbcNKIkelU=; b=kzlt/SZQFcjIPwUzQDPTLkvWpKub5P+af9/wHQ3dANjK1ySwr5YIrl0QIwiP3hOmkL 4b5WSRm4U+DBtzNugAG7c18+IzyM6Wnfll2H5DzomEdU8c1t5w5A5H4Rfe7S76IniY0D SfZ2MzwFK3h/UqoDGNIwlaQJOlw4FsDdGhOPOX5U1JsB+Xt3Funpkw4vlN0Bv3IKSRoE iR2F3xaRzBFfI5U0f8iQ4L+hFiftHTbOaRXIbJ/66djLIB7d8je/ETOpB+rEl9jZxdHF +XOSvmNcdOzXqPylEHVT69bq9lPf0M01J0q4pq6pEydhlJC+W/GJxCjGL7kNGhYRVeNg Rg4Q== X-Gm-Message-State: AOAM533aes6icojwVH7//kXFlcfdkP2shEZ367pr+ExJIvqdr3AEi1tv w6hD/w3CMyrNuSJHg6PuXDDo3g== X-Google-Smtp-Source: ABdhPJztd8X9PkPqPBt+mePsOqrRf4zvhkmVEscAYT44kSObS9rUvpzOQV1mbjxPBgBiGbWIkNUOYg== X-Received: by 2002:a37:b307:: with SMTP id c7mr35592806qkf.307.1596050477865; Wed, 29 Jul 2020 12:21:17 -0700 (PDT) Received: from eggly.attlocal.net (172-10-233-147.lightspeed.sntcca.sbcglobal.net. [172.10.233.147]) by smtp.gmail.com with ESMTPSA id s128sm2212336qkd.108.2020.07.29.12.21.12 (version=TLS1 cipher=ECDHE-ECDSA-AES128-SHA bits=128/128); Wed, 29 Jul 2020 12:21:16 -0700 (PDT) Date: Wed, 29 Jul 2020 12:21:11 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Pengfei Li cc: akpm@linux-foundation.org, bmt@zurich.ibm.com, dledford@redhat.com, willy@infradead.org, vbabka@suse.cz, kirill.shutemov@linux.intel.com, jgg@ziepe.ca, alex.williamson@redhat.com, cohuck@redhat.com, daniel.m.jordan@oracle.com, dbueso@suse.de, jglisse@redhat.com, jhubbard@nvidia.com, ldufour@linux.ibm.com, Liam.Howlett@oracle.com, peterz@infradead.org, cl@linux.com, jack@suse.cz, rientjes@google.com, walken@google.com, hughd@google.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH 2/2] mm, util: account_locked_vm() does not hold mmap_lock In-Reply-To: <20200726080224.205470-2-fly@kernel.page> Message-ID: References: <20200726080224.205470-1-fly@kernel.page> <20200726080224.205470-2-fly@kernel.page> User-Agent: Alpine 2.11 (LSU 23 2013-08-11) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-Rspamd-Queue-Id: 7269D180F8B86 X-Spamd-Result: default: False [0.00 / 100.00] X-Rspamd-Server: rspam02 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 Sun, 26 Jul 2020, Pengfei Li wrote: > Since mm->locked_vm is already an atomic counter, account_locked_vm() > does not need to hold mmap_lock. I am worried that this patch, already added to mmotm, along with its 1/2 making locked_vm an atomic64, might be rushed into v5.9 with just that two-line commit description, and no discussion at all. locked_vm belongs fundamentally to mm/mlock.c, and the lock to guard it is mmap_lock; and mlock() has some complicated stuff to do under that lock while it decides how to adjust locked_vm. It is very easy to convert an unsigned long to an atomic64_t, but "atomic read, check limit and do stuff, atomic add" does not give the same guarantee as holding the right lock around it all. (At the very least, __account_locked_vm() in 1/2 should be changed to replace its atomic64_add by an atomic64_cmpxchg, to enforce the limit that it just checked. But that will be no more than lipstick on a pig, when the right lock that everyone else agrees upon is not being held.) Now, it can be argued that our locked_vm and pinned_vm maintenance is so random and deficient, and too difficult to keep right across a sprawl of drivers, that we should just be grateful for those that do volunteer to subject themselves to RLIMIT_MEMLOCK limitation, and never mind if it's a little racy. And it may well be that all those who have made considerable efforts in the past to improve the situation, have more interesting things to devote their time to, and would prefer not to get dragged back here. But let's at least give this a little more visibility, and hope to hear opinions one way or the other from those who care. Hugh > > Signed-off-by: Pengfei Li > --- > drivers/vfio/vfio_iommu_type1.c | 8 ++------ > mm/util.c | 15 +++------------ > 2 files changed, 5 insertions(+), 18 deletions(-) > > diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c > index 78013be07fe7..53818fce78a6 100644 > --- a/drivers/vfio/vfio_iommu_type1.c > +++ b/drivers/vfio/vfio_iommu_type1.c > @@ -376,12 +376,8 @@ static int vfio_lock_acct(struct vfio_dma *dma, long npage, bool async) > if (!mm) > return -ESRCH; /* process exited */ > > - ret = mmap_write_lock_killable(mm); > - if (!ret) { > - ret = __account_locked_vm(mm, abs(npage), npage > 0, dma->task, > - dma->lock_cap); > - mmap_write_unlock(mm); > - } > + ret = __account_locked_vm(mm, abs(npage), npage > 0, > + dma->task, dma->lock_cap); > > if (async) > mmput(mm); > diff --git a/mm/util.c b/mm/util.c > index 473add0dc275..320fdd537aea 100644 > --- a/mm/util.c > +++ b/mm/util.c > @@ -424,8 +424,7 @@ void arch_pick_mmap_layout(struct mm_struct *mm, struct rlimit *rlim_stack) > * @task: task used to check RLIMIT_MEMLOCK > * @bypass_rlim: %true if checking RLIMIT_MEMLOCK should be skipped > * > - * Assumes @task and @mm are valid (i.e. at least one reference on each), and > - * that mmap_lock is held as writer. > + * Assumes @task and @mm are valid (i.e. at least one reference on each). > * > * Return: > * * 0 on success > @@ -437,8 +436,6 @@ int __account_locked_vm(struct mm_struct *mm, unsigned long pages, bool inc, > unsigned long locked_vm, limit; > int ret = 0; > > - mmap_assert_write_locked(mm); > - > locked_vm = atomic64_read(&mm->locked_vm); > if (inc) { > if (!bypass_rlim) { > @@ -476,17 +473,11 @@ EXPORT_SYMBOL_GPL(__account_locked_vm); > */ > int account_locked_vm(struct mm_struct *mm, unsigned long pages, bool inc) > { > - int ret; > - > if (pages == 0 || !mm) > return 0; > > - mmap_write_lock(mm); > - ret = __account_locked_vm(mm, pages, inc, current, > - capable(CAP_IPC_LOCK)); > - mmap_write_unlock(mm); > - > - return ret; > + return __account_locked_vm(mm, pages, inc, > + current, capable(CAP_IPC_LOCK)); > } > EXPORT_SYMBOL_GPL(account_locked_vm); > > -- > 2.26.2