Re: [PATCH v2] mmap.2: document new MAP_FIXED_SAFE flag

[John Hubbard]

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 ,
>