[PATCH] mmap.2: document new MAP_FIXED_SAFE flag

[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 <jhubbard@xxxxxxxxxx>]
Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
---
 man2/mmap.2 | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/man2/mmap.2 b/man2/mmap.2
index 385f3bfd5393..923bbb290875 100644
--- a/man2/mmap.2
+++ b/man2/mmap.2
@@ -225,6 +225,22 @@ will fail.
 Because requiring a fixed address for a mapping is less portable,
 the use of this option is discouraged.
 .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 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
@@ -449,6 +465,12 @@ 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

-- 
Michal Hocko
SUSE Labs