Re: [PATCH 2/2 v4] [BZ #10283] localedef: align fixed maps to SHMLBA

[Mike Frysinger <vapier at gentoo dot org>]

On Tuesday 11 June 2013 13:40:23 Roland McGrath wrote:
> > On Monday 03 June 2013 15:14:56 Roland McGrath wrote:
> > > > +#include <sys/shm.h>
> > > > +#define MAP_FIXED_ALIGNMENT SHMLBA
> > > 
> > > This is the Linux-specific definition.  The generic file should just
> > > use page size, or perhaps the macro should be defined such that the
> > > code (or at least the compiler) can tell statically that there is no
> > > need for additional operations at all when the requirement is no
> > > larger than page size.
> > 
> > in the discussion on this matter earlier, there were multiple points
> > raised.  it isn't unique to Linux (at least Solaris does it), and if this
> > generally the sane default, then having it be the generic default makes
> > sense.  especially since it doesn't really hurt when it's larger, and
> > it's safer.
> 
> It is not as yet the specified GNU API.  I don't think it should be,
> either.

if the hurd guys care, they can add their own that does whatever they need it 
to.  this behavior wouldn't break them.

even then, nothing has changed for them.  the locale code today is explicitly 
rounding to page sizes.  the default SHMLBA define does that too:
bits/shm.h:#define SHMLBA       (__getpagesize ())

as well as the gnu header:
sysdeps/gnu/bits/shm.h:#define SHMLBA       (__getpagesize ())

so with this new header in the place i'm proposing, the only targets that 
generate different code are the few Linux arches that define SHMLBA larger than 
getpagesize.

after all, the issue we're talking about here isn't really founded in the 
Linux port, but from the hardware itself.  it arises due to aliasing issues 
with hardware memory maps.  so i don't see how the hurd guys could even avoid 
it if they ever get around to doing a port for sparc/arm/ia64/hppa/etc...
-mike

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