Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751173AbdLMJba (ORCPT ); Wed, 13 Dec 2017 04:31:30 -0500 Received: from mail-pl0-f65.google.com ([209.85.160.65]:42107 "EHLO mail-pl0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752291AbdLMJbY (ORCPT ); Wed, 13 Dec 2017 04:31:24 -0500 X-Google-Smtp-Source: ACJfBouHJbPTicYXVPKraqMKsQYhpgteQ1stZZLYtMGEj+HRu4PpHvGrXVUVv5+6ONztFeSX8LdePg== From: Michal Hocko To: Michael Kerrisk Cc: linux-api@vger.kernel.org, Khalid Aziz , Michael Ellerman , Andrew Morton , Russell King - ARM Linux , Andrea Arcangeli , , LKML , linux-arch@vger.kernel.org, Florian Weimer , John Hubbard , Matthew Wilcox , Michal Hocko Subject: [PATCH 1/2] mmap.2: document new MAP_FIXED_SAFE flag Date: Wed, 13 Dec 2017 10:31:09 +0100 Message-Id: <20171213093110.3550-1-mhocko@kernel.org> X-Mailer: git-send-email 2.15.0 In-Reply-To: <20171213092550.2774-1-mhocko@kernel.org> References: <20171213092550.2774-1-mhocko@kernel.org> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1852 Lines: 53 From: Michal Hocko 4.16+ kernels offer a new MAP_FIXED_SAFE flag which allows the caller to atomicaly probe for a given address range. [wording heavily updated by John Hubbard ] Signed-off-by: Michal Hocko --- man2/mmap.2 | 22 ++++++++++++++++++++++ 1 file changed, 22 insertions(+) diff --git a/man2/mmap.2 b/man2/mmap.2 index a5a8eb47a263..02d391697ce6 100644 --- a/man2/mmap.2 +++ b/man2/mmap.2 @@ -227,6 +227,22 @@ in mind that the exact layout of a process' memory map is allowed to change significantly between kernel versions, C library versions, and operating system releases. .TP +.BR MAP_FIXED_SAFE " (since Linux 4.16)" +Similar to MAP_FIXED with respect to the +.I +addr +enforcement, but different in that MAP_FIXED_SAFE 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. It indicates to the kernel virtual memory system that the mapping @@ -451,6 +467,12 @@ 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 , -- 2.15.0