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From: John Hubbard <jhubbard@nvidia.com>
To: Michal Hocko <mhocko@kernel.org>,
	Michael Kerrisk <mtk.manpages@gmail.com>
Cc: linux-api@vger.kernel.org, Khalid Aziz <khalid.aziz@oracle.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Andrew Morton <akpm@linux-foundation.org>,
	Russell King - ARM Linux <linux@armlinux.org.uk>,
	Andrea Arcangeli <aarcange@redhat.com>,
	linux-mm@kvack.org, LKML <linux-kernel@vger.kernel.org>,
	linux-arch@vger.kernel.org, Florian Weimer <fweimer@redhat.com>
Subject: Re: [PATCH v2] mmap.2: document new MAP_FIXED_SAFE flag
Date: Thu, 30 Nov 2017 10:31:12 -0800
Message-ID: <1a96e274-a986-12b7-6e47-fc6355cbf637@nvidia.com> (raw)
In-Reply-To: <20171130082405.b77eknaiblgmpa4s@dhcp22.suse.cz>

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@suse.com>
> 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@nvidia.com>]
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
>  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 ,
> 

  reply index

Thread overview: 44+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-11-29 14:42 [PATCH 0/2] mm: introduce MAP_FIXED_SAFE Michal Hocko
2017-11-29 14:42 ` [PATCH 1/2] " Michal Hocko
2017-12-06  5:15   ` Michael Ellerman
2017-12-06  9:27     ` Michal Hocko
2017-12-06 10:02       ` Michal Hocko
2017-12-07 12:07   ` Pavel Machek
2017-11-29 14:42 ` [PATCH 2/2] fs, elf: drop MAP_FIXED usage from elf_map Michal Hocko
2017-11-29 17:45   ` Khalid Aziz
2017-11-29 14:45 ` [PATCH] mmap.2: document new MAP_FIXED_SAFE flag Michal Hocko
2017-11-30  3:16   ` John Hubbard
2017-11-30  8:23     ` Michal Hocko
2017-11-30  8:24   ` [PATCH v2] " Michal Hocko
2017-11-30 18:31     ` John Hubbard [this message]
2017-11-30 18:39       ` Michal Hocko
2017-11-29 15:13 ` [PATCH 0/2] mm: introduce MAP_FIXED_SAFE Rasmus Villemoes
     [not found]   ` <b154b794-7a8b-995e-0954-9234b9446b31-rjjw5hvvQKZaa/9Udqfwiw@public.gmane.org>
2017-11-29 15:50     ` Michal Hocko
2017-11-29 22:15   ` Kees Cook
2017-11-29 22:12 ` Kees Cook
2017-11-29 22:25 ` Kees Cook
     [not found]   ` <CAGXu5jLa=b2HhjWXXTQunaZuz11qUhm5aNXHpS26jVqb=G-gfw-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2017-11-30  6:58     ` Michal Hocko
2017-12-01 15:26       ` Cyril Hrubis
2017-12-06  4:51         ` Michael Ellerman
2017-12-06  4:54           ` Matthew Wilcox
2017-12-06  7:03             ` Matthew Wilcox
2017-12-06  7:33               ` John Hubbard
     [not found]                 ` <5f4fc834-274a-b8f1-bda0-5bcddc5902ed-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
2017-12-06  7:35                   ` Florian Weimer
2017-12-06  8:06                     ` John Hubbard
     [not found]                       ` <27ee1755-76d8-f086-5760-9c973b31108a-DDmLM1+adcrQT0dZR+AlfA@public.gmane.org>
2017-12-06  8:54                         ` Florian Weimer
     [not found]               ` <20171206070355.GA32044-PfSpb0PWhxZc2C7mugBRk2EX/6BAtgUQ@public.gmane.org>
2017-12-07  5:46                 ` Michael Ellerman
2017-12-07 19:14                   ` Kees Cook
     [not found]                     ` <CAGXu5jLWRQn6EaXEEvdvXr+4gbiJawwp1EaLMfYisHVfMiqgSA-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2017-12-07 19:57                       ` Matthew Wilcox
2017-12-08  8:33                         ` Michal Hocko
     [not found]                           ` <20171208083315.GR20234-2MMpYkNvuYDjFM9bn6wA6Q@public.gmane.org>
2017-12-08 20:13                             ` Kees Cook
     [not found]                               ` <CAGXu5j+VupGmKEEHx-uNXw27Xvndu=0ObsBqMwQiaYPyMGD+vw-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2017-12-08 20:57                                 ` Matthew Wilcox
2017-12-08 11:08                         ` Michael Ellerman
2017-12-08 14:27                           ` Pavel Machek
2017-12-08 20:31                             ` Cyril Hrubis
2017-12-08 20:47                             ` Florian Weimer
2017-12-08 14:33                           ` David Laight
2017-12-06  4:50       ` Michael Ellerman
     [not found]         ` <87zi6we9z2.fsf-W0DJWXSxmBNbyGPkN3NxC2scP1bn1w/D@public.gmane.org>
2017-12-06  7:33           ` Rasmus Villemoes
     [not found]             ` <a3b3129a-2626-a65e-59b0-68aada523723-rjjw5hvvQKZaa/9Udqfwiw@public.gmane.org>
2017-12-06  9:08               ` Michal Hocko
2017-12-07  0:19                 ` Kees Cook
2017-12-07  1:08                   ` John Hubbard

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