On 11/30/2017 12:24 AM, Michal Hocko wrote: > Updated version based on feedback from John. > --- > From ade1eba229b558431581448e7d7838f0e1fe2c49 Mon Sep 17 00:00:00 2001 > From: Michal Hocko <mhocko@xxxxxxxx> > Date: Wed, 29 Nov 2017 15:32:08 +0100 > Subject: [PATCH] mmap.2: document new MAP_FIXED_SAFE flag > > 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 ...and now I've created my own typo: please make that "fall back to a" (the "to" was missing). Sorry about the churn. It turns out that the compiler doesn't catch these. :) thanks, John Hubbard > +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 , >
