From: Ken Brown <kbrown@cornell.edu>
To: cygwin-developers@cygwin.com
Subject: Re: page_size vs allocation_granularity
Date: Wed, 22 Jul 2020 07:36:47 -0400 [thread overview]
Message-ID: <12dc9944-cf6c-1a56-f85d-e4e58a2d45d5@cornell.edu> (raw)
In-Reply-To: <20200722083327.GR16360@calimero.vinschen.de>
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.
Yaakov's patch cites
https://cygwin.com/ml/cygwin/2017-05/msg00350.html
I'll take a look.
Ken
next prev parent reply other threads:[~2020-07-22 11:36 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-21 22:40 Ken Brown
2020-07-22 8:33 ` Corinna Vinschen
2020-07-22 8:47 ` Corinna Vinschen
2020-07-22 11:36 ` Ken Brown [this message]
2020-07-22 16:42 ` Ken Brown
2020-07-22 18:35 ` Ken Brown
2020-07-22 18:48 ` Corinna Vinschen
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