Messages in this thread | | | Subject | Re: [PATCH v2] mmap.2: MAP_FIXED updated documentation | From | John Hubbard <> | Date | Mon, 4 Dec 2017 18:14:18 -0800 |
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On 12/04/2017 02:55 AM, Cyril Hrubis wrote: > Hi! > I know that we are not touching the rest of the existing description for > MAP_FIXED however the second sentence in the manual page says that "addr > must be a multiple of the page size." Which however is misleading as > this is not enough on some architectures. Code in the wild seems to > (mis)use SHMLBA for aligment purposes but I'm not sure that we should > advise something like that in the manpages. > > So what about something as: > > "addr must be suitably aligned, for most architectures multiple of page > size is sufficient, however some may impose additional restrictions for > page mapping addresses." >
Hi Cyril,
Right, so I've been looking into this today, and I think we can go a bit further than that, even. The kernel, as far back as the *original* git commit in 2005, implements mmap on ARM by requiring that the address is aligned to SHMLBA:
arch/arm/mm/mmap.c:50:
if (flags & MAP_FIXED) { if (aliasing && flags & MAP_SHARED && (addr - (pgoff << PAGE_SHIFT)) & (SHMLBA - 1)) return -EINVAL; return addr; }
So, given that this has been the implementation for the last 12+ years (and probably the whole time, in fact), I think we can be bold enough to use this wording for the second sentence of MAP_FIXED:
"addr must be a multiple of SHMLBA (<sys/shm.h>), which in turn is either the system page size (on many architectures) or a multiple of the system page size (on some architectures)."
What do you think?
thanks, John Hubbard NVIDIA
> Which should at least hint the reader that this is architecture specific. >
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