mmap failing with MAP_FIXED

mmap failing with MAP_FIXED

[David Allsopp]

This program fails at the second mmap call with EINVAL:

  #include <stdio.h>
  #include <sys/mman.h>
  #include <error.h>

  int main (void) {
    void * mem;
    /* Reserve 256MB address space for the minor heaps */
    mem = mmap(0, 268439552, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
    if (mem == MAP_FAILED)
      error(1, 0, "Reservation failed");
    /* Commit the first 2MB heap */
    if (mmap(mem, 2097152, PROT_READ | PROT_WRITE, MAP_PRIVATE |
MAP_ANONYMOUS | MAP_FIXED, -1, 0) == MAP_FAILED)
      error(1, 0, "Commit failed");
  }

Is this something that's expected to fail for Cygwin, or a bug? The example
is extracted from OCaml 5.0's runtime, which reserves an area of address
space and then commits chunks of it as required. The above snippet comes
from the Linux side, on Windows we're using VirtualAlloc with PAGE_NOACCESS
to reserve the address space and then VirtualAlloc with MEM_COMMIT and
PAGE_READWRITE to commit smaller portions of it.

Is there a way to do that with Cygwin's mmap?

Thanks,


David

Re: mmap failing with MAP_FIXED

[gs-cygwin.com]

On Fri, Jul 01, 2022 at 05:33:31PM +0100, David Allsopp wrote:
> This program fails at the second mmap call with EINVAL:
> 
>   #include <stdio.h>
>   #include <sys/mman.h>
>   #include <error.h>
> 
>   int main (void) {
>     void * mem;
>     /* Reserve 256MB address space for the minor heaps */
>     mem = mmap(0, 268439552, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
>     if (mem == MAP_FAILED)
>       error(1, 0, "Reservation failed");
>     /* Commit the first 2MB heap */
>     if (mmap(mem, 2097152, PROT_READ | PROT_WRITE, MAP_PRIVATE |
> MAP_ANONYMOUS | MAP_FIXED, -1, 0) == MAP_FAILED)
>       error(1, 0, "Commit failed");
>   }
> 
> Is this something that's expected to fail for Cygwin, or a bug? The example
> is extracted from OCaml 5.0's runtime, which reserves an area of address
> space and then commits chunks of it as required. The above snippet comes
> from the Linux side, on Windows we're using VirtualAlloc with PAGE_NOACCESS
> to reserve the address space and then VirtualAlloc with MEM_COMMIT and
> PAGE_READWRITE to commit smaller portions of it.
> 
> Is there a way to do that with Cygwin's mmap?

Have you tried 'msync()' with MS_ASYNC flag as an alternative approach?

Cheers, Glenn

Re: mmap failing with MAP_FIXED

[Corinna Vinschen]

On Jul  1 17:33, David Allsopp wrote:
> This program fails at the second mmap call with EINVAL:
> 
>   #include <stdio.h>
>   #include <sys/mman.h>
>   #include <error.h>
> 
>   int main (void) {
>     void * mem;
>     /* Reserve 256MB address space for the minor heaps */
>     mem = mmap(0, 268439552, PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
>     if (mem == MAP_FAILED)
>       error(1, 0, "Reservation failed");
>     /* Commit the first 2MB heap */
>     if (mmap(mem, 2097152, PROT_READ | PROT_WRITE, MAP_PRIVATE |
> MAP_ANONYMOUS | MAP_FIXED, -1, 0) == MAP_FAILED)
>       error(1, 0, "Commit failed");
>   }
> 
> Is this something that's expected to fail for Cygwin, or a bug?

Depends on the point of view, I guess.  PROT_NONE pages are not just
reserved pages in the Windows sense.  mmap'ed pages are always commited
and PROT_NONE is just a page protection.  Using mmap to change the page
protection of already commited memory is not implemented.  To change
this, use mprotect.


Corinna