From: Ken Brown <kbrown@cornell.edu>
To: cygwin@cygwin.com
Subject: Re: Atomic mmap replacement
Date: Mon, 19 Feb 2018 22:33:00 -0000 [thread overview]
Message-ID: <e5417923-d1cf-1f2e-ef9d-fd50eef60581@cornell.edu> (raw)
In-Reply-To: <20180219171914.GA3619@calimero.vinschen.de>
On 2/19/2018 12:19 PM, Corinna Vinschen wrote:
> On Feb 19 08:22, Ken Brown wrote:
>> On 2/19/2018 4:00 AM, Corinna Vinschen wrote:
>>> On Feb 17 22:37, Ken Brown wrote:
>>>> Some code in emacs wants to reserve a chunk of address space with a big
>>>> PROT_NONE anonymous mapping, and then carve it up into separate mappings
>>>> associated to segments of a file. This fails on Cygwin. Here's a test case
>>>> that illustrates the problem:
>>>> [...]
>>> Several limitations in the Windows kernel disallow this:
>>>
>>> - It doesn't allow to unmap parts of a map, only the entire map as a
>>> whole.
>>> Cygwin has a workaround: If you unmap parts of a map it just keeps
>>> track of this and sets the protection of the affected pages to
>>> PAGE_NOACCESS. In case of anonymous mappings, it even recycles them
>>> potentially for other mappings.
>>>
>>> - It also disallows to re-map any allocated or mapped mamory for another
>>> purpose.
>>>
>>> So this part of the POSIX specs for mmap:
>>>
>>> "The mapping established by mmap() shall replace any previous mappings
>>> for those whole pages containing any part of the address space of the
>>> process starting at pa and continuing for len bytes"
>>>
>>> can't be implemented with Windows means.
>>>
>>> The only workaround possible would be to handle this *exact* scenario as
>>> a special case in Cygwin's mmap: If the new mapping falls in the middle
>>> of an existing mapping and if the original mapping was an anonymous
>>> mapping with PROT_NONE page protection, then
>
> On second thought, we *could* do this, if the pages have been mmapped
> before(*). Unfortunately this would require a *major* revamp of the
> page handling in mmap. We would have to keep the mapping of every
> single 64K page separate.
>
> I.e., requesting a file mapping of 256K at offset 0 on the POSIX level
> would have to be handled as four Windows file mappings under the hood:
>
> 1. a 64K file mapping at offset 0
> 2. a 64K file mapping at offset 65536
> 3. a 64K file mapping at offset 131072
> 4. a 64K file mapping at offset 196608
>
> A request to mmap another 64K page to the third mapping in this example
> could then be done by unmapping the third mapping and replace it with
> the requested mapping.
>
> I'm not sure this is feasible. It would complicate and slow down the
> code especially for big mappings; one call to NtCreateSection and one to
> NtMapViewOfSection per 64K page, plus the overhead of making sure that
> all mappings are in the right, sequential order in memory. Plus the
> overhead of having to remap a lot more mappings in forked children. The
> "Cygwin is slow" meme would get another interesting facet :}
That doesn't sound great.
In the meantime, the problem was solved on the emacs side by doing what
you suggested in your previous email:
> - unmap the old mapping
> - remap the unaffected parts as separate anonymous mapping
> - map the affected parts for the requested file mapping
But in the emacs application it's simpler, because there are no
unaffected parts, so step 2 can be skipped.
Thanks.
Ken
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next prev parent reply other threads:[~2018-02-19 22:33 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-02-18 3:37 Ken Brown
2018-02-19 9:01 ` Corinna Vinschen
2018-02-19 13:22 ` Ken Brown
2018-02-19 17:19 ` Corinna Vinschen
2018-02-19 22:33 ` Ken Brown [this message]
2018-02-20 10:31 ` Corinna Vinschen
2018-02-20 1:04 ` John Hood
2018-02-20 10:47 ` Corinna Vinschen
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