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    SubjectRe: [PATCH] mmap.2: document new MAP_FIXED_NOREPLACE flag
    On Wed, Apr 11, 2018 at 2:04 PM,  <mhocko@kernel.org> wrote:
    > From: Michal Hocko <mhocko@suse.com>
    >
    > 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@nvidia.com>]
    > Signed-off-by: Michal Hocko <mhocko@suse.com>
    > ---
    > 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.

    This still looks wrong to me. There are legitimate uses for MAP_FIXED,
    and for most users of MAP_FIXED that I'm aware of, MAP_FIXED_NOREPLACE
    wouldn't work while MAP_FIXED works perfectly well.

    MAP_FIXED is for when you have already reserved the targeted memory
    area using another VMA; MAP_FIXED_NOREPLACE is for when you haven't.
    Please don't make it sound as if MAP_FIXED is always wrong.

    > +.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.

    Maybe add something like ", and MAP_FIXED_NOREPLACE was specified"? I
    think most manpages explicitly document which error conditions can be
    triggered by which flags.

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