Atomic mmap replacement

[Ken Brown]

On 2/19/2018 4:00 AM, Corinna Vinschen wrote:
> On Feb 17 22:37, Ken Brown wrote:
>> Some code in emacs wants to reserve a chunk of address space with a big
>> PROT_NONE anonymous mapping, and then carve it up into separate mappings
>> associated to segments of a file.  This fails on Cygwin.  Here's a test case
>> that illustrates the problem:
>>
>> $ truncate -s 64k foo
>>
>> $ cat mmap_test.c
>> #include <stdio.h>
>> #include <stdlib.h>
>> #include <fcntl.h>
>> #include <sys/mman.h>
>>
>> const size_t page_size = 64 * 1024;
>>
>> int
>> main ()
>> {
>>    void *mem = mmap (NULL, 2 * page_size,
>>                      PROT_NONE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
>>    if (mem == MAP_FAILED)
>>      {
>>        perror ("mmap");
>>        exit (1);
>>      }
>>    int fd = open ("foo", O_RDONLY);
>>    void *res = mmap (mem, page_size, PROT_READ | PROT_WRITE,
>>                      MAP_PRIVATE | MAP_FIXED, fd, 0);
>>    if (res == MAP_FAILED)
>>      {
>>        perror ("mmap");
>>        exit (2);
>>      }
>> }
>>
>> $ gcc mmap_test.c
>>
>> $ ./a
>> mmap: Invalid argument
>>
>> $ echo $?
>> 2
>>
>> Is this a bug, or is it simply a limitation of Cygwin's mmap?  If the
>> latter, is there a simple workaround?
> 
> Several limitations in the Windows kernel disallow this:
> 
> - It doesn't allow to unmap parts of a map, only the entire map as a
>    whole.
>    
>    Cygwin has a workaround: If you unmap parts of a map it just keeps
>    track of this and sets the protection of the affected pages to
>    PAGE_NOACCESS.  In case of anonymous mappings, it even recycles them
>    potentially for other mappings.
> 
> - It also disallows to re-map any allocated or mapped mamory for another
>    purpose.
> 
> So this part of the POSIX specs for mmap:
> 
>    "The mapping established by mmap() shall replace any previous mappings
>     for those whole pages containing any part of the address space of the
>     process starting at pa and continuing for len bytes"
> 
> can't be implemented with Windows means.
> 
> The only workaround possible would be to handle this *exact* scenario as
> a special case in Cygwin's mmap:  If the new mapping falls in the middle
> of an existing mapping and if the original mapping was an anonymous
> mapping with PROT_NONE page protection, then
> 
> - unmap the old mapping
> - remap the unaffected parts as separate anonymous mapping
> - map the affected parts for the requested file mapping
> 
> This is pretty complicated and I'm not hot on implementing it.  If it's
> really required we can take a look of course.

Thanks, Corinna.  I'll take this information back to the emacs list.

Ken

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