From: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
To: Jann Horn <jannh@google.com>, Michal Hocko <mhocko@kernel.org>
Cc: mtk.manpages@gmail.com, John Hubbard <jhubbard@nvidia.com>,
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: document new MAP_FIXED_NOREPLACE flag
Date: Thu, 12 Apr 2018 10:10:18 +0200 [thread overview]
Message-ID: <6c2df5a8-b33a-915f-f220-da9a6ad08049@gmail.com> (raw)
In-Reply-To: <CAG48ez2wYqxJEHgZCz5g6ZYBY4_qDcYWSGAErC8pUzmrW62rug@mail.gmail.com>
Hi Jann,
On 04/11/2018 06:40 PM, Jann Horn wrote:
> On Wed, Apr 11, 2018 at 6:36 PM, Michal Hocko <mhocko@kernel.org> wrote:
>> On Wed 11-04-18 17:37:46, Jann Horn wrote:
>>> 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.
>>
>> Well, this was suggested by John. I think, nobody is objecting that
>> MAP_FIXED has legitimate usecases. The above text just follows up on
>> the previous section which emphasises the potential memory corruption
>> problems and it suggests that a new flag is safe with that regards.
>>
>> If you have specific wording that would be better I am open for changes.
>
> I guess I'd probably also want to change the previous text; so I
> should probably send a followup patch once this one has landed.
Okay -- I'm ready to take that piece now. Please send me a patch!
Cheers,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
next prev parent reply other threads:[~2018-04-12 8:10 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-04-11 12:04 [PATCH] mmap.2: document new MAP_FIXED_NOREPLACE flag mhocko
2018-04-11 15:37 ` Jann Horn
2018-04-11 16:36 ` Michal Hocko
2018-04-11 16:40 ` Jann Horn
2018-04-11 17:06 ` Michal Hocko
2018-04-12 8:09 ` Michael Kerrisk (man-pages)
2018-04-12 8:10 ` Michael Kerrisk (man-pages) [this message]
2018-04-11 20:36 ` John Hubbard
2018-04-12 8:04 ` Michael Kerrisk (man-pages)
2018-04-12 8:21 ` Michal Hocko
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