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Date: Tue, 17 Dec 2024 17:38:35 +0100
To: cygwin AT cygwin DOT com
Subject: Re: Atomic mmap replacement
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From: Corinna Vinschen via Cygwin <cygwin AT cygwin DOT com>
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Hi Ken,

On Dec 17 10:34, Ken Brown via Cygwin wrote:
> On 12/16/2024 2:58 PM, Ken Brown via Cygwin wrote:
> > 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.
> While looking at this, I came across a bug in the use of
> mmap_list::try_map(): On success, the recycled pages are given the same
> protection they had as part of the original mmap_record instead of the
> protection requested in the mmap call.  This is done in
> mmap_record::map_pages.

Thanks for catching!

> Would you like a separate patch to fix this, or should I just fix it as part
> of the bigger project?

Ideally a separate patch.  If you see a chance to send it to cygwin-patches
this week, we can even merge it into 3.5.5, which I'm planning to release
end of this week (after that --> vacation :)))


Thanks,
Corinna

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