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Subject[PATCH 1/2] mmap.2: document new MAP_FIXED_SAFE flag
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
+.BR MAP_FIXED_SAFE " (since Linux 4.16)"
+Similar to MAP_FIXED with respect to the
+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
+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.
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
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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