Re: [PATCH] mmap.2: MAP_FIXED is no longer discouraged

[Matthew Wilcox]

On Fri, Dec 01, 2017 at 06:16:26PM -0800, john.hubbard@xxxxxxxxx wrote:
> MAP_FIXED has been widely used for a very long time, yet the man
> page still claims that "the use of this option is discouraged".

I think we should continue to discourage the use of this option, but
I'm going to include some of your text in my replacement paragraph ...

-Because requiring a fixed address for a mapping is less portable,
-the use of this option is discouraged.
+The use of this option is discouraged because it forcibly unmaps any
+existing mapping at that address.  Programs which use this option need
+to be aware that their memory map may change significantly from one run to
+the next, depending on library versions, kernel versions and random numbers.
+In a threaded process, checking the existing mappings can race against
+a new dynamic library being loaded, for example in response to another
+thread making a library call which causes a PAM module to be loaded.

(I don't love this text, in particular "PAM module".  I'm going off to
use the ATM machine now.  Please edit.)