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Wed, 11 Apr 2018 11:38:09 -0400 Received: from mail-oi0-f44.google.com ([209.85.218.44]:42400 "EHLO mail-oi0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753268AbeDKPiH (ORCPT ); Wed, 11 Apr 2018 11:38:07 -0400 X-Google-Smtp-Source: AIpwx4/9c7L104ayP99Fu2eJWVVL0wDMic+YtpPS6jEPyNce0MPkr8qs6X4A7zEdDOJ5sT1wyEl8Q6RB00TyilpS18U= MIME-Version: 1.0 In-Reply-To: <20180411120452.1736-1-mhocko@kernel.org> References: <20180411120452.1736-1-mhocko@kernel.org> From: Jann Horn Date: Wed, 11 Apr 2018 17:37:46 +0200 Message-ID: Subject: Re: [PATCH] mmap.2: document new MAP_FIXED_NOREPLACE flag To: Michal Hocko Cc: Michael Kerrisk , John Hubbard , Andrew Morton , Linux-MM , LKML , Linux API , Michal Hocko Content-Type: text/plain; charset="UTF-8" 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: 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. > +.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. Maybe add something like ", and MAP_FIXED_NOREPLACE was specified"? I think most manpages explicitly document which error conditions can be triggered by which flags.