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

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Message-ID: <efb6eae4-7f30-42c3-0efe-0ab5fbf0fdb4@nvidia.com>
Date:   Mon, 4 Dec 2017 18:14:18 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Cyril Hrubis <chrubis@...e.cz>
CC:     Michael Kerrisk <mtk.manpages@...il.com>,
        linux-man <linux-man@...r.kernel.org>,
        <linux-api@...r.kernel.org>, Michael Ellerman <mpe@...erman.id.au>,
        <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
        <linux-arch@...r.kernel.org>, Jann Horn <jannh@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH v2] mmap.2: MAP_FIXED updated documentation

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.
>