mmap failure [was: cygrunsrv + sshd + rsync = 20 times too slow -- throttled?]

[Ken Brown]

On 9/6/2021 4:54 PM, Peter Dons Tychsen wrote:
> Hi there,
> 
> On Mon, 2021-09-06 at 14:40 -0400, Ken Brown via Cygwin wrote:
>>> No, wait.  I get what you say.  The optimzation settings of the test
>>> case should have no influence on the code inside the DLL.  That
>>> doesn't
>>> make sense for sure.  However, I ran the testcase under GDB, I could
>>> reproduce the issue, and I could fix it by setting mmap_ext.Reserved
>>> = 0;
>>> Go figure!
>>
>> I don't get it, but I can confirm that the problem is fixed.
> 
> That sounds a bit like a voodoo fix, that could quickly regress again.
> 
> Here is my 2 cents:
> 
> Currently the mmap_ext structure is setup like this:
> 
>   215       MEM_EXTENDED_PARAMETER mmap_ext = {
>   216         .Type = MemExtendedParameterAddressRequirements,
>   217         .Pointer = (PVOID) &mmap_req
>   218       };
> 
> This means that all other entries in the struct are zero at
> initialization as described here:
> https://en.cppreference.com/w/c/language/struct_initialization
> 
> So if you set "mmap_ext.Reserved = 0" again after that its a double
> failure.

You're looking at the wrong source code.  The bug didn't occur until the code 
was changed to do the following:

       /* g++ 11.2 workaround: don't use initializer */
       MEM_EXTENDED_PARAMETER mmap_ext;
       mmap_ext.Type = MemExtendedParameterAddressRequirements;
       mmap_ext.Pointer = (PVOID) &mmap_req;

This left mmap_ext.Reserved uninitialized, which Corinna has now fixed.

Ken