Re: [PATCH 0/2] mm: introduce MAP_FIXED_SAFE

[Michael Ellerman]

Matthew Wilcox <willy@xxxxxxxxxxxxx> writes:

> 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@xxxxxxx> 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.

MAP_REQUIRED doesn't immediately grab me, but I don't actively dislike
it either :)

What about MAP_AT_ADDR ?

It's short, and says what it does on the tin. The first argument to mmap
is actually called "addr" too.

cheers