page_size vs allocation_granularity

[Ken Brown]

On 7/22/2020 4:33 AM, Corinna Vinschen wrote:
> On Jul 21 18:40, Ken Brown via Cygwin-developers wrote:
>> Hi Corinna,
>>
>> I'm curious about the design decision that causes sysconf(_SC_PAGESIZE) to
>> return wincap.allocation_granularity() rather than wincap.page_size().
>> Changing this would improve Linux compatibility, I think, but maybe it would
>> have some bad consequences that I'm not aware of.
> 
> It was a long and hard process to move from 4K to 64K pagesize, with
> lots of loaded discussions.  The Cygwin mailing list archives will
> show a lot of this in the 200X years.
> 
> It was the only way to make mmap 99% POSIX-conformant.  Consider, for
> instance this:
> 
>    pagesize = sysconf(_SC_PAGESIZE);
>    addr = mmap (NULL, pagesize, PROT_READ | PROT_WRITE,
> 	       MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
>    addr2 = mmap (addr + pagesize, pagesize, PROT_READ | PROT_WRITE,
> 		MAP_FIXED | MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
> 
> On Windows, this fails with pagesize = 4K, but it works with pagesize =
> 64K, because of that idiotic Windows allocation granularity.  Almost
> all POSIX expectations are automagically fixed by using the granularity
> as pagesize in a POSIX sense.
> 
> There's only one problem left:  While you can only allocate usefully in
> 64K steps, the size of the memory area allocated for a file is only 4K
> aligned, thus leaving the remainder of the 64K block unmapped.
> 
> This problem could be fixed back in 32 bit times by adding the
> AT_ROUND_TO_PAGE mapping.  Very unfortunately, the 64 bit Windows
> designer decided to keep the braindead 64K allocation granularity
> but to drop the AT_ROUND_TO_PAGE flag, thus removing the only chance
> to make this single situation POSIX-compatible as well.
> 
>> I'm asking because in my recent fooling around with php, I noticed that
>> Yaakov had to apply the following Cygwin-specific patch to avoid a crash:
> 
> It would be nice to learn what kind of crash that was.

Here's a better reference than the one I gave in my previous reply, which 
actually explains what's going on:

   https://sourceware.org/pipermail/cygwin/2017-May/232562.html

> If php reads or writes in the remainder of the block constituting EOF,
> or tries to change page protection, shit happens.  Every time, a process
> stabs into the EOF block following the last valid 4K block, it results
> in a STATUS_ACCESS_VIOLATION which in turn calls
> mmap_is_attached_or_noreserve().  While this situation can be
> recognized, I don't see a way to fix this from the processes POV.

So that's exactly what happens when php maps a file whose size is a multiple of 
4K but not a multiple of 64K.  It expects that there is a zero-filled region 
beyond EOF that it can safely read from.

Ken