Re: [PATCH] mmap.2: document new MAP_FIXED_NOREPLACE flag

[John Hubbard]

On 04/11/2018 08:37 AM, Jann Horn wrote:
> On Wed, Apr 11, 2018 at 2:04 PM,  <mhocko@xxxxxxxxxx> wrote:
>> From: Michal Hocko <mhocko@xxxxxxxx>
>>
>> 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@xxxxxxxxxx>]
>> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
>> ---
>> 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.

That's a nice summary, I hope it shows up in your upcoming patch. I recall
that we went back and forth, trying to find a balance of explaining
this feature, without providing overly-elaborate examples (which I tend
toward).

> Please don't make it sound as if MAP_FIXED is always wrong.
> 

Agreed.

thanks,
-- 
John Hubbard
NVIDIA