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Re: mmap(MAP_FIXED) vs mprotect

On Apr 24 08:03, Ryan Johnson wrote:
> Hi all,
> I'm trying to port a linux program that uses mmap to implement a
> growable array; the ideas is to mmap(PROT_NONE, MAP_NORESERVE) a
> chunk of address space (corresponding to the maximum array size) and
> then call mmap(PROT_READ|PROT_WRITE, MAP_FIXED) to allocate actual
> memory in the "blank" region. This works well in Linux but fails
> with EINVAL in cygwin.
> My code aligns all sizes up to 2MB boundaries, so it's not a 64kB
> boundary problem. My code reports the failing call as:
> >22 Invalid argument addr=0xffdb0000, sz=2097152
> A peek in /proc/self/maps confirms that the address is correct:
> >FFDB0000-FFFB0000 ===p 00000000 0000:0000 0
> Oddly, trying to map in blank pages in with mprotect succeeds on
> cygwin but fails with ENOMEM on linux...
> Am I missing something here, or is this just a place where different
> behavior between the two platforms is a fact of life? Which version
> is the posixly "correct" way to reserve a chunk of address space and
> later back it with actual memory?

There is no POSIXly correct way to do that since MAP_NORESERVE is a
non-POSIX extension in Linux as well as in Cygwin.

The general idea of MAP_NORESERVE is to make sure that we get as much
memory as requested, but to use only as much memory as is required.
On Linux MAP_NORESERVE only performs bookkeeping but doesn't change
the state of the memory, so a later mmap works.  On Cygwin MAP_NORESERVE
uses the Windows way of handling this requirements, so in contrast to
what the name of the option suggests, Cygwin actually *reserves* space
but does not *commit* it.  Cygwin's mmap can't handle this, but you can
commit pages by using mprotect or by simply peeking or poking into
the address space.  This raises a SEGV, and the exception handling code
will then commit the page you peeked or poked.

Having said that, we *could* also change mmap to handle this scenario
gracefully as well.


Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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