linux-kernel - Question regarding mmap() with MAP

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Message-ID: <BANLkTin5M2-3puDxxo5Cxs5qHkS7vGeH8w@mail.gmail.com>
Date:	Tue, 14 Jun 2011 06:17:13 +0900
From:	Jongman Heo <jongman.heo@...il.com>
To:	linux-kernel@...r.kernel.org
Subject: Question regarding mmap() with MAP_FIXED flag

Dear all,

Please let me ask a dumb question.

I use following mmap call, to get the same virtual address with given
physical address.
And when reading the mmap-ed memory, it seems that the virtual address
correctly points to the physical address.

  ptr = mmap((void*)0x400000, getpagesize(), PROT_READ, MAP_PRIVATE |
MAP_FIXED , mem_fd, 0x400000);

  where mem_fd is the file descriptor of /dev/mem.

And using memmap boot arguments, 4M RAM location is not used by Linux.
The area is written by bootloader. (.. memmap=exactmap memmap=64k@0
memmap=3M@1M memmap=1019M@5M)

It works, but I'm afraid that the possiblity of mmap() failure with
MAP_FIXED flag.
man page of mmap states following:

   MAP_FIXED  Do not select a different address than the one
specified.  If the memory  region  speci-
                  fied  by  start  and  len overlaps pages of any
existing mapping(s), then the overlapped
                  part of the existing mapping(s) will be discarded.
If the specified address  cannot  be
                  used, mmap() will fail.  If MAP_FIXED is specified,
start must be a multiple of the page
                  size.  Use of this option is discouraged.

Q1. What does the existing mapping mean?
Another mmap mapping by any other user space process? Or it means that
the virtual address (0x400000 in my case) is already acquired/used by
others (kernel, process, or whatever)?

Q2. When can't the specified address be used?

Q3. Is there any solution such that mmap with MAP_FIXED being not
failed but always succeed?

Thanks in advance,
Jongman Heo.
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