Re: sys_mmap2 on different architectures

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Paul Mackerras wrote:

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).


ACK on that. I was looking at old kernel sources (2.6.14-rc timeframe), and I guess that one only supported 4K 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:

That's what I thought, too, but it doesn't seem to match reality.

This is what I've found so far: (64-bit architectures excluded)

	arm		- N/A (PAGE_SHIFT == 12)
	arm26		- MMAP2_PAGE_SHIFT == 12
	cris		- MMAP2_PAGE_SHIFT == PAGE_SHIFT (13)
	frv		- MMAP2_PAGE_SHIFT == 12
	h8300		- N/A (PAGE_SHIFT == 12)
	i386		- N/A (PAGE_SHIFT == 12)
	m32r		- N/A (PAGE_SHIFT == 12)
	m68k		- MMAP2_PAGE_SHIFT == PAGE_SHIFT (variable)
	mips		- MMAP2_PAGE_SHIFT == PAGE_SHIFT (variable)
	parisc		- MMAP2_PAGE_SHIFT == 12
	ppc		- MMAP2_PAGE_SHIFT == 12
	s390		- N/A (PAGE_SHIFT == 12)
	sh		- N/A (PAGE_SHIFT == 12)
	sparc		- MMAP2_PAGE_SHIFT == 12
	v850		- N/A (PAGE_SHIFT == 12)
	xtensa		- N/A (PAGE_SHIFT == 12)

So, excluding 64-bit architectures, we have 3 architectures which expect getpagesize() to be used, 5 which expect the constant value 12, and 8 which get the same result either way. In effect, we have a system call with subtly different semantics across architectures, and there isn't any clear distinction each way.

This is something I don't enjoy about Linux :-/
	
/* 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).

Changing the user-kernel ABI is bad. I'm just trying to get to the bottom with what the API actually *IS*.

	-hpa
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