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Re: Broken autoconf mmap test
- From: Christopher Faylor <cgf-use-the-mailinglist-please at cygwin dot com>
- To: cygwin-developers at cygwin dot com
- Date: Thu, 24 Mar 2011 12:21:25 -0400
- Subject: Re: Broken autoconf mmap test
- References: <20110323091921.GB10657@calimero.vinschen.de> <4D89D4AA.firstname.lastname@example.org> <20110323132622.GM31220@calimero.vinschen.de> <4D89FEC7.email@example.com> <20110323154421.GN31220@calimero.vinschen.de> <4D8A308F.firstname.lastname@example.org> <4D8A3206.email@example.com> <4D8A3329.firstname.lastname@example.org> <20110323181507.GV31220@calimero.vinschen.de> <20110324095945.GD31220@calimero.vinschen.de>
- Reply-to: cygwin-developers at cygwin dot com
On Thu, Mar 24, 2011 at 10:59:45AM +0100, Corinna Vinschen wrote:
>On Mar 23 19:15, Corinna Vinschen wrote:
>> On Mar 23 11:51, Eric Blake wrote:
>> > I'm still debating if cygwin is compliant and the autoconf test is
>> > exploiting undefined behavior... this is tricky stuff to get right.
>> Cygwin tries to be as compliant as possible, and it tries to implement
>> expectations which are typical for some Unix systems even if they are
>> not covered by the standards.
>> And yes, the autoconf test tests defined behaviour, see the SUSv4 mmap
>> man page:
>For the records and, hopefully, even for discussion if somebody feels
>a bit of pity with me.
>I've been mulling over this problem on and off for a long time. I just
>don't see a solution for 64 bit systems by just using the OS calls as
>they are defined. AT_ROUND_TO_PAGE is not supported, AT_EXTENDABLE_FILE
>changes the file size.
>So I was thinking of an entirely different way to implement mmap's on
>real files: Instead of really mapping them, mmap just generates an
>In case of MAP_PRIVATE, anonymous maps are implemented using
>VirtualAlloc. The size of the allocated space is always a multiple of
>64K. After creating it, the file content is simply copied over.
>Full stop. Easy.
>In case of MAP_SHARED, anonymous maps are created using an actual
>file mapping (NtCreateSection/NtMapViewOfSection). For files, mmap
>would still simply copy over the file content.
>The problem starts if the map has been opened using PROT_WRITE. In that
>case we have to write the changed pages back to the file. Fortunately
>the only times you can rely on the changes being written back to the
>file are msync, munmap, and process exit.
>But that's the tricky part. How do we know a page has been written
>Easy solution: Always write all of the mapping to the file. Would that
>be possible? This sounds wrong to me. IIUC, msync should only write
>back the changes, not the entire content of the mapping. It would
>potentially overwrite changes made in the file by other means.
>Starting with Windows 2000 there's a method to keep track of pages which
>have been written to: Call VirtualAlloc with the MEM_WRITE_WATCH flag
>set, and then use GetWriteWatch to find the changed pages.
>Unfortunately the MEM_WRITE_WATCH flag works only for VirtualAlloc'ated
>memory, not for section maps created with MapViewOfFile, so it's not
>usable in this scenario. We can't use VirtualAlloc for the MAP_SHARED
>case, otherwise the changes in memory are not visible to other processes
>sharing the map.
>The hard way, apparently. Always set the page protection to
>PAGE_READONLY. If the process writes to it, an exception occurs. Catch
>the exception, set the page protection to PAGE_READWRITE and return from
>the exception so the caller can write again. When the changes are to be
>written back to the file, only write back the pages with PAGE_READWRITE
>protection. If this is called from msync, reset the page protection to
>Apart from being complicated and error prone, there are a couple of
>problems I can see:
>- The protection set by mmap and mprotect doesn't match the actual
> protection. So we need to keep track of the desired protection
> per page. This takes one byte per page on the cygheap. So far we
> only needed 1 bit per page.
>- It's darn tricky to make this thread-safe.
>- In all three possible solutions above: What if the original file
> handle used in the mmap call has been closed and the file permissions
> have been changed in the meantime so that the process does not have
> write permissions anymore?
- Isn't it going to be really slow? I guess if it only happens once
per page it won't be that bad but still: ouch.