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Date: Sat, 14 Dec 2024 19:00:42 -0500
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Subject: Re: Atomic mmap replacement
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From: Ken Brown via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Ken Brown <kbrown AT cornell DOT edu>
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Hi Corinna,

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.

[...]

> Several limitations in the Windows kernel disallow this:

[...]

> - 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.

I'm returning to this very old thread because of come up against another 
application that wants to allocate a big block of memory and then 
allocate pieces of it later.  I've looked at the documentation of 
VirtualAlloc, and it seems that this should be possible:

    VirtualAlloc cannot reserve a reserved page. It can commit a page
    that is already committed. This means you can commit a range of
    pages, regardless of whether they have already been committed, and
    the function will not fail.

    You can use VirtualAlloc to reserve a block of pages and then make
    additional calls to VirtualAlloc to commit individual pages from
    the reserved block. This enables a process to reserve a range of
    its virtual address space without consuming physical storage until
    it is needed.

The attached test case illustrates this.  Do you think it's feasible to 
modify mmap to take advantage of this?  If you try to mmap a block 
that's inside an already allocated block, I would want mmap to call 
VirtualAlloc with MEM_COMMIT but not MEM_RESERVE, and this should succeed.

If you think this is feasible, I would be willing to work on it.  But in 
that case I would appreciate some suggestions on how to implement it, 
since I'm not yet very familiar with the mmap code.

Ken
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