From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wr0-f198.google.com (mail-wr0-f198.google.com [209.85.128.198]) by kanga.kvack.org (Postfix) with ESMTP id 35BAD6B0009 for ; Thu, 12 Apr 2018 04:10:21 -0400 (EDT) Received: by mail-wr0-f198.google.com with SMTP id 38so1631720wrv.8 for ; Thu, 12 Apr 2018 01:10:21 -0700 (PDT) Received: from mail-sor-f65.google.com (mail-sor-f65.google.com. [209.85.220.65]) by mx.google.com with SMTPS id 2sor993665wmz.19.2018.04.12.01.10.19 for (Google Transport Security); Thu, 12 Apr 2018 01:10:19 -0700 (PDT) Subject: Re: [PATCH] mmap.2: document new MAP_FIXED_NOREPLACE flag References: <20180411120452.1736-1-mhocko@kernel.org> <20180411163631.GL23400@dhcp22.suse.cz> From: "Michael Kerrisk (man-pages)" Message-ID: <6c2df5a8-b33a-915f-f220-da9a6ad08049@gmail.com> Date: Thu, 12 Apr 2018 10:10:18 +0200 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Jann Horn , Michal Hocko Cc: mtk.manpages@gmail.com, John Hubbard , Andrew Morton , Linux-MM , LKML , Linux API Hi Jann, On 04/11/2018 06:40 PM, Jann Horn wrote: > On Wed, Apr 11, 2018 at 6:36 PM, Michal Hocko wrote: >> On Wed 11-04-18 17:37:46, Jann Horn wrote: >>> On Wed, Apr 11, 2018 at 2:04 PM, wrote: >>>> From: Michal Hocko >>>> >>>> 4.17+ kernels offer a new MAP_FIXED_NOREPLACE flag which allows the caller to >>>> atomicaly probe for a given address range. >>>> >>>> [wording heavily updated by John Hubbard ] >>>> Signed-off-by: Michal Hocko >>>> --- >>>> Hi, >>>> Andrew's sent the MAP_FIXED_NOREPLACE to Linus for the upcoming merge >>>> window. So here we go with the man page update. >>>> >>>> man2/mmap.2 | 27 +++++++++++++++++++++++++++ >>>> 1 file changed, 27 insertions(+) >>>> >>>> diff --git a/man2/mmap.2 b/man2/mmap.2 >>>> index ea64eb8f0dcc..f702f3e4eba2 100644 >>>> --- a/man2/mmap.2 >>>> +++ b/man2/mmap.2 >>>> @@ -261,6 +261,27 @@ Examples include >>>> and the PAM libraries >>>> .UR http://www.linux-pam.org >>>> .UE . >>>> +Newer kernels >>>> +(Linux 4.17 and later) have a >>>> +.B MAP_FIXED_NOREPLACE >>>> +option that avoids the corruption problem; if available, MAP_FIXED_NOREPLACE >>>> +should be preferred over MAP_FIXED. >>> >>> This still looks wrong to me. There are legitimate uses for MAP_FIXED, >>> and for most users of MAP_FIXED that I'm aware of, MAP_FIXED_NOREPLACE >>> wouldn't work while MAP_FIXED works perfectly well. >>> >>> MAP_FIXED is for when you have already reserved the targeted memory >>> area using another VMA; MAP_FIXED_NOREPLACE is for when you haven't. >>> Please don't make it sound as if MAP_FIXED is always wrong. >> >> Well, this was suggested by John. I think, nobody is objecting that >> MAP_FIXED has legitimate usecases. The above text just follows up on >> the previous section which emphasises the potential memory corruption >> problems and it suggests that a new flag is safe with that regards. >> >> If you have specific wording that would be better I am open for changes. > > I guess I'd probably also want to change the previous text; so I > should probably send a followup patch once this one has landed. Okay -- I'm ready to take that piece now. Please send me a patch! Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/