linux-kernel - Re: [PATCH 0/2] mm: introduce MAP_FIXED

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Message-ID: <5f4fc834-274a-b8f1-bda0-5bcddc5902ed@nvidia.com>
Date:   Tue, 5 Dec 2017 23:33:58 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Matthew Wilcox <willy@...radead.org>,
        Michael Ellerman <mpe@...erman.id.au>
CC:     Cyril Hrubis <chrubis@...e.cz>, Michal Hocko <mhocko@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Linux API <linux-api@...r.kernel.org>,
        Khalid Aziz <khalid.aziz@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Florian Weimer <fweimer@...hat.com>,
        Abdul Haleem <abdhalee@...ux.vnet.ibm.com>,
        Joel Stanley <joel@....id.au>
Subject: Re: [PATCH 0/2] mm: introduce MAP_FIXED_SAFE

On 12/05/2017 11:03 PM, Matthew Wilcox wrote:
> On Tue, Dec 05, 2017 at 08:54:35PM -0800, Matthew Wilcox wrote:
>> On Wed, Dec 06, 2017 at 03:51:44PM +1100, Michael Ellerman wrote:
>>> Cyril Hrubis <chrubis@...e.cz> writes:
>>>
>>>> Hi!
>>>>>> MAP_FIXED_UNIQUE
>>>>>> MAP_FIXED_ONCE
>>>>>> MAP_FIXED_FRESH
>>>>>
>>>>> Well, I can open a poll for the best name, but none of those you are
>>>>> proposing sound much better to me. Yeah, naming sucks...
>>>>
>>>> Given that MAP_FIXED replaces the previous mapping MAP_FIXED_NOREPLACE
>>>> would probably be a best fit.
>>>
>>> Yeah that could work.
>>>
>>> I prefer "no clobber" as I just suggested, because the existing
>>> MAP_FIXED doesn't politely "replace" a mapping, it destroys the current
>>> one - which you or another thread may be using - and clobbers it with
>>> the new one.
>>
>> It's longer than MAP_FIXED_WEAK :-P
>>
>> You'd have to be pretty darn strong to clobber an existing mapping.
> 
> I think we're thinking about this all wrong.  We shouldn't document it as
> "This is a variant of MAP_FIXED".  We should document it as "Here's an
> alternative to MAP_FIXED".
> 
> So, just like we currently say "exactly one of MAP_SHARED or MAP_PRIVATE",
> we could add a new paragraph saying "at most one of MAP_FIXED or
> MAP_REQUIRED" and "any of the following values".
> 
> Now, we should implement MAP_REQUIRED as having each architecture
> define _MAP_NOT_A_HINT, and then #define MAP_REQUIRED (MAP_FIXED |
> _MAP_NOT_A_HINT), but that's not information to confuse users with.
> 
> Also, that lets us add a third option at some point that is Yet Another
> Way to interpret the 'addr' argument, by having MAP_FIXED clear and
> _MAP_NOT_A_HINT set.
> 
> I'm not set on MAP_REQUIRED.  I came up with some awful names
> (MAP_TODDLER, MAP_TANTRUM, MAP_ULTIMATUM, MAP_BOSS, MAP_PROGRAM_MANAGER,
> etc).  But I think we should drop FIXED from the middle of the name.
> 

In that case, maybe:

    MAP_EXACT

? ...because that's the characteristic behavior. It doesn't clobber, but
you don't need to say that in the name, now that we're not including
_FIXED_ in the middle.

thanks,
John Hubbard
NVIDIA