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Date:   Wed, 29 Nov 2017 15:45:24 +0100
From:   Michal Hocko <>
To:     Michael Kerrisk <>
Cc:, Khalid Aziz <>,
        Michael Ellerman <>,
        Andrew Morton <>,
        Russell King - ARM Linux <>,
        Andrea Arcangeli <>, <>,
        LKML <>,,
        Florian Weimer <>,
        John Hubbard <>,
        Michal Hocko <>
Subject: [PATCH] mmap.2: document new MAP_FIXED_SAFE flag

From: Michal Hocko <>

4.16+ kernels offer a new MAP_FIXED_SAFE flag which allows to atomicaly
probe for a given address range.

Signed-off-by: Michal Hocko <>
 man2/mmap.2 | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/man2/mmap.2 b/man2/mmap.2
index 385f3bfd5393..622a7000de83 100644
--- a/man2/mmap.2
+++ b/man2/mmap.2
@@ -225,6 +225,18 @@ will fail.
 Because requiring a fixed address for a mapping is less portable,
 the use of this option is discouraged.
+.B MAP_FIXED_SAFE (since 4.16)
+Similar to MAP_FIXED wrt. to the
+enforcement except it never clobbers a colliding mapped range and rather fail with
+in such a case. This flag can therefore be used as a safe and atomic probe for the
+the specific address range. Please note that older kernels which do not recognize
+this flag can fallback to the hint based implementation and map to a different
+location. Any backward compatible software should therefore check the returning
+address with the given one.
 This flag is used for stacks.
 It indicates to the kernel virtual memory system that the mapping
@@ -449,6 +461,12 @@ is not a valid file descriptor (and
 was not set).
+range covered by
+.IR addr , 
+.IR length
+is clashing with an existing mapping.
 We don't like
 .IR addr ,

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