Atomic mmap replacement

[Ken Brown]

On 12/16/2024 8:32 AM, Corinna Vinschen via Cygwin wrote:
> Right now, mmaping with PROT_NONE and then re-mmaping with PROT_WRITE
> doesn't work.  Cygwin implements PROT_NONE not as MAP_RESERVE, but as
> MEM_COMMIT with PAGE_NOACCESS.  mmap() doesn't check if the requested
> pages are already allocated with PAGE_NOACCESS and then succeeds while
> in fact just changing the page protection.  This is basically what
> you want.  Right now, you'd have to call mprotect() instead.
> 
> With anonymous mappings only, this is all just adding a bit of code to
> mmap() to do what mprotect() does in this case.

[...]

> So only anonymous mappings would be possible, assuming we tweak mmap()
> to check if the old mapping was anonymous either and then allow to
> just change the page protection, as if mprotect has been called.
> 
> And, funny enough, something pretty similar already exists in mmap().
> See mmap.cc, line 1051 and the mmap_list::try_map() method.  Right
> now it only checks if an anonymous mapping has been partially unmapped
> and can be recycled.  But it could be improved by allowing to recycle
> the anonymous mapping either way, as long as the new mapping is also
> anonymous and the SHARED/PRIVATE flags match.
Thanks!  This looks doable.  I hope to get back to you with a patch in 
the not-too-distant future.

Ken