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

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Date:   Tue, 5 Dec 2017 23:03:55 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     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>,
        John Hubbard <jhubbard@...dia.com>,
        Abdul Haleem <abdhalee@...ux.vnet.ibm.com>,
        Joel Stanley <joel@....id.au>
Subject: Re: [PATCH 0/2] mm: introduce MAP_FIXED_SAFE

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.