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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Mon, 25 Feb 2019 21:15:34 -0000 Received: from b06wcsmtp001.portsmouth.uk.ibm.com (b06wcsmtp001.portsmouth.uk.ibm.com [9.149.105.160]) by b06cxnps4075.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x1PLFXV332243892 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Mon, 25 Feb 2019 21:15:34 GMT Received: from b06wcsmtp001.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id DC04CA4062; Mon, 25 Feb 2019 21:15:33 +0000 (GMT) Received: from b06wcsmtp001.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 0BDA3A4054; Mon, 25 Feb 2019 21:15:31 +0000 (GMT) Received: from rapoport-lnx (unknown [9.148.204.243]) by b06wcsmtp001.portsmouth.uk.ibm.com (Postfix) with ESMTPS; Mon, 25 Feb 2019 21:15:30 +0000 (GMT) Date: Mon, 25 Feb 2019 23:15:28 +0200 From: Mike Rapoport To: Peter Xu Cc: Jerome Glisse , 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 References: <20190212025632.28946-1-peterx@redhat.com> <20190212025632.28946-24-peterx@redhat.com> <20190221183653.GV2813@redhat.com> <20190225085846.GE13653@xz-x1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190225085846.GE13653@xz-x1> User-Agent: Mutt/1.5.24 (2015-08-30) X-TM-AS-GCONF: 00 x-cbid: 19022521-0016-0000-0000-0000025AC7EF X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19022521-0017-0000-0000-000032B5271F Message-Id: <20190225211528.GF10454@rapoport-lnx> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-02-25_11:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=848 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1902250151 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Feb 25, 2019 at 04:58:46PM +0800, Peter Xu wrote: > 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. I think (c) is "wp range and wake the thread", and (d) is "wp and DONT wake". > > > > > > > > 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). Waking up the thread after the last region is write-protected would make sense. Not much savings for lots of ranges, though. > > > > > range.start = uffdio_wp.range.start; > > > range.len = uffdio_wp.range.len; > > > wake_userfault(ctx, &range); > > Thanks, > > -- > Peter Xu > -- Sincerely yours, Mike.