Re: Atomic mmap replacement

[Corinna Vinschen <corinna-cygwin@cygwin.com>]

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On Feb 19 20:03, John Hood wrote:
> 
> 
> On 02/19/2018 12:19, Corinna Vinschen wrote:
> > 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 :}
> > 
> I work on an app that does something like this (but for other reasons, and
> it'll never be ported to Cygwin).
> 
> I'm also dubious, but I'll point out that it'd probably be reasonable to do
> this only on regions that are mapped PROT_NONE initially, other regions
> could work as they do now.

As far as I'm concerned, the problem is that we collect special cases
which raise the number of lines of code, getting more complicated and
less well debuggable, but still only work in some cases.

Assuming we'd do this at all, doing this per page *all* the time would
certainly lead to simpler code.  The big change affects the bookkeeping
code in the first place.  Right now we have a list of files (anonympous
mappings are one file) with a list of mappings per file attached.  In
theory, to support scenarios like this, the bookkeeping structure should
be changed to a hash or list of per-page structs.  So we would look at
maps starting at the page address, rather than starting at the file
descriptor the map is attached to.

I'm not opposed to change Cygwin's mmap this way, but it's a loooot of
work and there's so much to do in so limited a time...

If somebody would like to take a stab at this, I'd be pretty excited.
Fortunately the mmap code is collected in a single source file
winsup/cygwin/mmap.cc, so you only need limited insight in the Cygwin
code.  Help from my side is guaranteed.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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