linux-kernel - Re: [PATCH v2] mmap.2: MAP_FIXED updated documentation

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Date:   Sat, 9 Dec 2017 18:19:58 +0100
From:   Pavel Machek <pavel@....cz>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Cyril Hrubis <chrubis@...e.cz>, John Hubbard <jhubbard@...dia.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        linux-man <linux-man@...r.kernel.org>, linux-api@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>, linux-arch@...r.kernel.org,
        Jann Horn <jannh@...gle.com>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v2] mmap.2: MAP_FIXED updated documentation

On Thu 2017-12-07 15:02:21, Michal Hocko wrote:
> On Thu 07-12-17 13:58:05, Cyril Hrubis wrote:
> > Hi!
> > > >> (It does seem unfortunate that the man page cannot help the programmer
> > > >> actually write correct code here. He or she is forced to read the kernel
> > > >> implementation, in order to figure out the true alignment rules. I was
> > > >> hoping we could avoid that.)
> > > > 
> > > > It would be nice if we had this information exported somehere so that we
> > > > do not have to rely on per-architecture ifdefs.
> > > > 
> > > > What about adding MapAligment or something similar to the /proc/meminfo?
> > > > 
> > > 
> > > What's the use case you envision for that? I don't see how that would be
> > > better than using SHMLBA, which is available at compiler time. Because 
> > > unless someone expects to be able to run an app that was compiled for 
> > > Arch X, on Arch Y (surely that's not requirement here?), I don't see how
> > > the run-time check is any better.
> > 
> > I guess that some kind of compile time constant in uapi headers will do
> > as well, I'm really open to any solution that would expose this constant
> > as some kind of official API.
> 
> I am not sure this is really feasible. It is not only a simple alignment
> thing. Look at ppc for example (slice_get_unmapped_area). Other
> architectures might have even more complicated rules e.g. arm and its
> cache_is_vipt_aliasing. Also this applies only on MAP_SHARED || file
> backed mappings.
> 
> I would really leave dogs sleeping... Trying to document all this in the
> man page has chances to confuse more people than it has chances to help
> those who already know all these nasty details.

You don't have to provide all the details, but warning that there's arch-
specific magic would be nice...
								Pavel

(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html