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Date:      Wed, 22 Jan 2003 22:43:41 -0500 (EST)
From:      Garrett Wollman <wollman@lcs.mit.edu>
To:        Matthew Dillon <dillon@apollo.backplane.com>
Cc:        <arch@FreeBSD.ORG>
Subject:   Re: getsysfd() patch #1 (Re: Virtual memory question) 
Message-ID:  <200301230343.h0N3hfXt044969@khavrinen.lcs.mit.edu>
In-Reply-To: <200301230259.h0N2xt9R017635@apollo.backplane.com>
References:  <20030122173229.D46974-100000@mail.chesapeake.net> <200301222249.h0MMn9e5016697@apollo.backplane.com> <200301222323.h0MNN7co043532@khavrinen.lcs.mit.edu> <200301230259.h0N2xt9R017635@apollo.backplane.com>

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<<On Wed, 22 Jan 2003 18:59:55 -0800 (PST), Matthew Dillon <dillon@apollo.backplane.com> said:

>     I have no idea what you are talking about here.  We can do swap-backed 
>     anonymous shared memory using SysV shared memory (non descriptor-based),

No, SysV shared memory is emphatically not anonymous.  It does not use
the filesystem namespace, but it certainly does have a namespace, and
every SysV shared memory segment has a name in that namespace.  (A
particularly badly thought-out one at that, which causes all sorts of
resource leaks when programs terminate unexpectedly.)

>     We have no interface that has any capability to do what we want. 
>     I've explained to you what the problem is several times now in 
>     exruciating detail

No, you haven't explained a damn thing.  You haven't even stated the
problem at all.  Tell me, once and for all, WTF is wrong, technically,
with the shm_open interface.  Don't give me any NIH crap, I'm not
interested in hearing your theories on technical elegance or what not.
We have a standard interface.  Given a lack of a clear problem
statement, I can only conclude that the interface *can* do what you
seem to be saying you want it to do (it was good enough for SSWG-RT),
but that you are for some reason firmly attached to your pet interface
instead.  How the current implementation of shm_open() works is
entirely irrelevant; my desire is to replace it.

>     We have no descriptor-based
>     shared memory mechanism which uses swap as backing store.  It's that
>     simple.

>     If you believe that such a mechanism already exists then, by all means,
>     tell us all what it is.

You claim to have already implemented it.  I'm not interested in
reinventing your work on the VM side of this.  I just want to avoid
public exposure of a proprietary interface when a standard interface
would do the job just as well.

>     That's a terrible hack.  I would never implement that.

Why don't we make some more molehills into insurmountable obstacles?

>     I would far prefer to implement a 
>     high-level operations vector on descriptors that covers both mmap() and
>     ftruncate() (at a high level),

Great idea, you do that.  I don't see it as a show-stopper; it is easy
to recognize and undo the hacks later when a better implementation is
ready.

-GAWollman


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