Re: [PATCH v2] mmap.2: MAP_FIXED updated documentation

[John Hubbard <jhubbard@nvidia.com>]

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.
>