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Thu, 12 Apr 2018 04:04:18 -0400 Received: from mail-wm0-f67.google.com ([74.125.82.67]:36863 "EHLO mail-wm0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753032AbeDLIEL (ORCPT ); Thu, 12 Apr 2018 04:04:11 -0400 X-Google-Smtp-Source: AIpwx4/16c88+O/2GD/hnbFtoFyBlkkWs+HpPvxVmyDZzoODpqVi+2nJcKkAmpg8CE8p/tKYvVlFXg== Cc: mtk.manpages@gmail.com, John Hubbard , Andrew Morton , linux-mm@kvack.org, LKML , linux-api@vger.kernel.org, Michal Hocko Subject: Re: [PATCH] mmap.2: document new MAP_FIXED_NOREPLACE flag To: mhocko@kernel.org References: <20180411120452.1736-1-mhocko@kernel.org> From: "Michael Kerrisk (man-pages)" Message-ID: <97504dda-4252-a150-e7b5-43fe587aa055@gmail.com> Date: Thu, 12 Apr 2018 10:04:06 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.6.0 MIME-Version: 1.0 In-Reply-To: <20180411120452.1736-1-mhocko@kernel.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-api-owner@vger.kernel.org X-Mailing-List: linux-api@vger.kernel.org X-getmail-retrieved-from-mailbox: INBOX X-Mailing-List: linux-kernel@vger.kernel.org List-ID: Hello Michal, On 04/11/2018 02:04 PM, mhocko@kernel.org 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 Thanks! I've applied your patch, and done a little tweaking. The results have already been pushed. Cheers Michael > --- > 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. > +.TP > +.BR MAP_FIXED_NOREPLACE " (since Linux 4.17)" > +Similar to MAP_FIXED with respect to the > +.I > +addr > +enforcement, but different in that MAP_FIXED_NOREPLACE never clobbers a pre-existing > +mapped range. If the requested range would collide with an existing > +mapping, then this call fails with > +.B EEXIST. > +This flag can therefore be used as a way to atomically (with respect to other > +threads) attempt to map an address range: one thread will succeed; all others > +will report failure. Please note that older kernels which do not recognize this > +flag will typically (upon detecting a collision with a pre-existing mapping) > +fall back to a "non-MAP_FIXED" type of behavior: they will return an address that > +is different than the requested one. Therefore, backward-compatible software > +should check the returned address against the requested address. > .TP > .B MAP_GROWSDOWN > This flag is used for stacks. > @@ -487,6 +508,12 @@ is not a valid file descriptor (and > .B MAP_ANONYMOUS > was not set). > .TP > +.B EEXIST > +range covered by > +.IR addr , > +.IR length > +is clashing with an existing mapping. > +.TP > .B EINVAL > We don't like > .IR addr , > -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ Linux/UNIX System Programming Training: http://man7.org/training/