Re: sys_mmap2 on different architectures

[Paul Mackerras <paulus@samba.org>]

H. Peter Anvin writes:

> I've looked through the code for sys_mmap2 on several architectures, and 
> it looks like some architectures plays by the "shift is always 12" rule, 
>   e.g. SPARC, and some expect userspace to actually obtain the page 
> size, e.g. PowerPC and MIPS.  On some architectures, e.g. x86 and ARM, 
> the point is moot since PAGE_SIZE is always 2^12.
> 
> a. Is this correct, or have I misunderstood the code?

PowerPC always uses 12, even if PAGE_SHIFT is 16 (i.e. for 64k
pages).

> b. If so, is this right, or is this a bug?  Right now both klibc and 
> µClibc consider the latter a bug.

Glibc seems to expect it to always be 12, according to this excerpt
from sysdeps/unix/sysv/linux/mmap64.c:

/* This is always 12, even on architectures where PAGE_SHIFT != 12.  */
# ifndef MMAP2_PAGE_SHIFT
#  define MMAP2_PAGE_SHIFT 12
# endif

I would be very reluctant to change the shift to be PAGE_SHIFT, since
that would be a change in the user/kernel ABI.  Of course, userspace
is still expected to make sure addresses and offsets are multiples of
the page size (and thus the offset argument to mmap2 has to be a
multiple of 16 if the page size is 64k).

Regards,
Paul.