Messages in this thread | | | Date | Thu, 23 Feb 2006 13:56:38 +1100 | From | Paul Mackerras <> | Subject | Re: sys_mmap2 on different architectures |
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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).
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