From: Michal Hocko <mhocko@xxxxxxxx> 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 <jhubbard@xxxxxxxxxx>] Signed-off-by: Michal Hocko <mhocko@xxxxxxxx> --- 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. +.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. +.TP .B EINVAL We don't like .IR addr , -- 2.16.3
