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[209.132.183.28]) by mx.google.com with ESMTPS id d9si1685570qve.12.2019.02.25.00.59.00 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 25 Feb 2019 00:59:00 -0800 (PST) Received-SPF: pass (google.com: domain of peterx@redhat.com designates 209.132.183.28 as permitted sender) client-ip=209.132.183.28; Authentication-Results: mx.google.com; spf=pass (google.com: domain of peterx@redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=peterx@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 030013003AFB; Mon, 25 Feb 2019 08:58:59 +0000 (UTC) Received: from xz-x1 (ovpn-12-105.pek2.redhat.com [10.72.12.105]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 1368560BE7; Mon, 25 Feb 2019 08:58:48 +0000 (UTC) Date: Mon, 25 Feb 2019 16:58:46 +0800 From: Peter Xu To: Jerome Glisse Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, David Hildenbrand , Hugh Dickins , Maya Gokhale , Pavel Emelyanov , Johannes Weiner , Martin Cracauer , Shaohua Li , Marty McFadden , Andrea Arcangeli , Mike Kravetz , Denis Plotnikov , Mike Rapoport , Mel Gorman , "Kirill A . Shutemov" , "Dr . David Alan Gilbert" Subject: Re: [PATCH v2 23/26] userfaultfd: wp: don't wake up when doing write protect Message-ID: <20190225085846.GE13653@xz-x1> References: <20190212025632.28946-1-peterx@redhat.com> <20190212025632.28946-24-peterx@redhat.com> <20190221183653.GV2813@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190221183653.GV2813@redhat.com> User-Agent: Mutt/1.10.1 (2018-07-13) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.45]); Mon, 25 Feb 2019 08:58:59 +0000 (UTC) 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 Thu, Feb 21, 2019 at 01:36:54PM -0500, Jerome Glisse wrote: > On Tue, Feb 12, 2019 at 10:56:29AM +0800, Peter Xu wrote: > > It does not make sense to try to wake up any waiting thread when we're > > write-protecting a memory region. Only wake up when resolving a write > > protected page fault. > > > > Signed-off-by: Peter Xu > > I am bit confuse here, see below. > > > --- > > fs/userfaultfd.c | 13 ++++++++----- > > 1 file changed, 8 insertions(+), 5 deletions(-) > > > > diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c > > index 81962d62520c..f1f61a0278c2 100644 > > --- a/fs/userfaultfd.c > > +++ b/fs/userfaultfd.c > > @@ -1771,6 +1771,7 @@ static int userfaultfd_writeprotect(struct userfaultfd_ctx *ctx, > > struct uffdio_writeprotect uffdio_wp; > > struct uffdio_writeprotect __user *user_uffdio_wp; > > struct userfaultfd_wake_range range; > > + bool mode_wp, mode_dontwake; > > > > if (READ_ONCE(ctx->mmap_changing)) > > return -EAGAIN; > > @@ -1789,18 +1790,20 @@ static int userfaultfd_writeprotect(struct userfaultfd_ctx *ctx, > > if (uffdio_wp.mode & ~(UFFDIO_WRITEPROTECT_MODE_DONTWAKE | > > UFFDIO_WRITEPROTECT_MODE_WP)) > > return -EINVAL; > > - if ((uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_WP) && > > - (uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_DONTWAKE)) [1] > > + > > + mode_wp = uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_WP; > > + mode_dontwake = uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_DONTWAKE; > > + > > + if (mode_wp && mode_dontwake) [2] > > return -EINVAL; > > I am confuse by the logic here. DONTWAKE means do not wake any waiting > thread right ? So if the patch header it seems to me the logic should > be: > if (mode_wp && !mode_dontwake) > return -EINVAL; This should be the most common case when we want to write protect a page (or a set of pages). I'll explain more details below... > > At very least this part does seems to mean the opposite of what the > commit message says. Let me paste the matrix to be clear on these flags: |------+-------------------------+------------------------------| | | dontwake=0 | dontwake=1 | |------+-------------------------+------------------------------| | wp=0 | (a) resolve pf, do wake | (b) resolve pf only, no wake | | wp=1 | (c) wp page range | (d) invalid | |------+-------------------------+------------------------------| Above check at [1] was checking against case (d) in the matrix. It is indeed an invalid condition because when we want to write protect a page we should not try to wake up any thread, so the donewake parameter is actually useless (we'll always do that). And above [2] is simply rewritting [1] with the new variables. > > > > > ret = mwriteprotect_range(ctx->mm, uffdio_wp.range.start, > > - uffdio_wp.range.len, uffdio_wp.mode & > > - UFFDIO_WRITEPROTECT_MODE_WP, > > + uffdio_wp.range.len, mode_wp, > > &ctx->mmap_changing); > > if (ret) > > return ret; > > > > - if (!(uffdio_wp.mode & UFFDIO_WRITEPROTECT_MODE_DONTWAKE)) { > > + if (!mode_wp && !mode_dontwake) { > > This part match the commit message :) Here is what the patch really want to change: before this patch we'll even call wake_userfault() below for case (c) while it doesn't really make too much sense IMHO. After this patch we'll only do the wakeup for (a,b). > > > range.start = uffdio_wp.range.start; > > range.len = uffdio_wp.range.len; > > wake_userfault(ctx, &range); Thanks, -- Peter Xu