Date: Mon, 11 Aug 2008 10:07:03 -0500
From: "Serge E. Hallyn" <serue@us.ibm.com>
To: Dave Hansen <dave@linux.vnet.ibm.com>
Cc: Arnd Bergmann <arnd@arndb.de>, containers@lists.linux-foundation.org,
       Theodore Tso <tytso@mit.edu>, linux-kernel@vger.kernel.org
Subject: Re: [RFC][PATCH 1/4] checkpoint-restart: general infrastructure
Message-ID: <20080811150703.GA25930@us.ibm.com>
References: <20080807224033.FFB3A2C1@kernel> <200808081146.54834.arnd@arndb.de> <1218221451.19082.36.camel@nimitz> <200808090013.41999.arnd@arndb.de> <1218234411.19082.58.camel@nimitz>
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Quoting Dave Hansen (dave@linux.vnet.ibm.com):
> On Sat, 2008-08-09 at 00:13 +0200, Arnd Bergmann wrote:
> > > I have to wonder if this is just a symptom of us trying to do this the
> > > wrong way.  We're trying to talk the kernel into writing internal gunk
> > > into a FD.  You're right, it is like a splice where one end of the pipe
> > > is in the kernel.
> > > 
> > > Any thoughts on a better way to do this?  
> > 
> > Maybe you can invert the logic and let the new syscalls create a file
> > descriptor, and then have user space read or splice the checkpoint
> > data from it, and restore it by writing to the file descriptor.
> > It's probably easy to do using anon_inode_getfd() and would solve this
> > problem, but at the same time make checkpointing the current thread
> > hard if not impossible.
> 
> Yeah, it does seem kinda backwards.  But, instead of even having to
> worry about the anon_inode stuff, why don't we just put it in a fs like
> everything else?  checkpointfs!

One reason is that I suspect that stops us from being able to send that
data straight to a pipe to compress and/or send on the network, without
hitting local disk.  Though if the checkpointfs was ram-based maybe not?

As Oren has pointed out before, passing in an fd means we can pass a
socket into the syscall.

Using the anon_inodes would also prevent that, but if it makes for a
cleaner overall solution then I'm not against considering either one
of course.

> I'm also really not convinced that putting the entire checkpoint in one
> glob is really the solution, either.  I mean, is system call overhead
> really a problem here?
> 
> -- Dave
> 
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