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From: Jann Horn <jannh@google.com>
To: Michal Hocko <mhocko@kernel.org>
Cc: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>,
	John Hubbard <jhubbard@nvidia.com>,
	linux-man <linux-man@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linux-MM <linux-mm@kvack.org>,
	lkml <linux-kernel@vger.kernel.org>,
	Linux API <linux-api@vger.kernel.org>
Subject: Re: [PATCH] mmap.2: MAP_FIXED is okay if the address range has been reserved
Date: Fri, 13 Apr 2018 18:17:36 +0200	[thread overview]
Message-ID: <CAG48ez1PdzMs8hkatbzSLBWYucjTc75o8ovSmeC66+e9mLvSfA@mail.gmail.com> (raw)
In-Reply-To: <CAG48ez3-xtmAt2EpRFR8GNKKPcsDsyg7XdwQ=D5w3Ym6w4Krjw@mail.gmail.com>

On Fri, Apr 13, 2018 at 6:05 PM, Jann Horn <jannh@google.com> wrote:
> On Fri, Apr 13, 2018 at 6:04 PM, Michal Hocko <mhocko@kernel.org> wrote:
>> On Fri 13-04-18 17:04:09, Jann Horn wrote:
>>> On Fri, Apr 13, 2018 at 8:49 AM, Michal Hocko <mhocko@kernel.org> wrote:
>>> > On Fri 13-04-18 08:43:27, Michael Kerrisk wrote:
>>> > [...]
>>> >> So, you mean remove this entire paragraph:
>>> >>
>>> >>               For cases in which the specified memory region has not been
>>> >>               reserved using an existing mapping,  newer  kernels  (Linux
>>> >>               4.17  and later) provide an option MAP_FIXED_NOREPLACE that
>>> >>               should be used instead; older kernels require the caller to
>>> >>               use addr as a hint (without MAP_FIXED) and take appropriate
>>> >>               action if the kernel places the new mapping at a  different
>>> >>               address.
>>> >>
>>> >> It seems like some version of the first half of the paragraph is worth
>>> >> keeping, though, so as to point the reader in the direction of a remedy.
>>> >> How about replacing that text with the following:
>>> >>
>>> >>               Since  Linux 4.17, the MAP_FIXED_NOREPLACE flag can be used
>>> >>               in a multithreaded program to avoid  the  hazard  described
>>> >>               above.
>>> >
>>> > Yes, that sounds reasonable to me.
>>>
>>> But that kind of sounds as if you can't avoid it before Linux 4.17,
>>> when actually, you just have to call mmap() with the address as hint,
>>> and if mmap() returns a different address, munmap() it and go on your
>>> normal error path.
>>
>> This is still racy in multithreaded application which is the main point
>> of the whole section, no?
>
> No, it isn't.

mmap() with a hint (without MAP_FIXED) will always non-racily allocate
a memory region for you or return an error code. If it does allocate a
memory region, it belongs to you until you deallocate it. It might be
at a different address than you requested - in that case you can
emulate MAP_FIXED_NOREPLACE by calling munmap() and treating it as an
error; or you can do something else with it.

MAP_FIXED_NOREPLACE is just a performance optimization.

  reply	other threads:[~2018-04-13 16:17 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-12 15:39 Jann Horn
2018-04-12 18:32 ` Michael Kerrisk (man-pages)
2018-04-12 18:33 ` John Hubbard
2018-04-12 18:37   ` Michael Kerrisk (man-pages)
2018-04-12 18:49     ` Jann Horn
2018-04-12 18:59       ` John Hubbard
2018-04-12 19:18         ` Jann Horn
2018-04-12 19:24           ` John Hubbard
2018-04-13  6:43             ` Michael Kerrisk (man-pages)
2018-04-13  6:49               ` Michal Hocko
2018-04-13 15:04                 ` Jann Horn
2018-04-13 16:04                   ` Michal Hocko
2018-04-13 16:05                     ` Jann Horn
2018-04-13 16:17                       ` Jann Horn [this message]
2018-04-16 10:07                         ` Michal Hocko
2018-04-16 13:55                           ` Jann Horn
2018-04-16 19:18                             ` Michal Hocko
2018-04-16 19:30                               ` Jann Horn
2018-04-16 19:57                                 ` Michal Hocko
2018-04-16 20:17                                   ` Jann Horn
2018-04-16 21:11                                     ` Michal Hocko
2018-04-16 21:12                                       ` Jann Horn
2018-04-17  6:23                                         ` Michal Hocko
2018-05-02 13:06                                       ` Michael Kerrisk (man-pages)

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