linux-kernel - Re: [RFC][PATCH 1/4] checkpoint-restart: general infrastructure

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Date:	Wed, 20 Aug 2008 17:40:51 -0400
From:	Oren Laadan <orenl@...columbia.edu>
To:	Pavel Machek <pavel@...e.cz>
CC:	"Serge E. Hallyn" <serue@...ibm.com>,
	containers@...ts.linux-foundation.org,
	Theodore Tso <tytso@....edu>, linux-kernel@...r.kernel.org,
	Arnd Bergmann <arnd@...db.de>,
	Dave Hansen <dave@...ux.vnet.ibm.com>
Subject: Re: [RFC][PATCH 1/4] checkpoint-restart: general infrastructure



Pavel Machek wrote:
> Hi!
> 
>>>>> 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.
> 
> If you do pass a socket, will it handle blocking correctly? Getting
> deadlocked task would be bad. What happens if I try to snapshot into
> /proc/self/fd/0 ? Or maybe restore from /proc/cmdline?

Hmmm... these are good points.

Keep in mind that our principal goal is to checkpoint a whole container,
rather then a task to checkpoint itself (which is a by-product). Of course
your comments apply to a whole container as well.

In both cases, I don't think that blocking on a socket is a problem; the
checkpointer will enter a TASK_INTERRUPTIBLE state. Where is the deadlock ?
Writing or reading to/from /proc/self/... likewise - the programmer must
understand the implications, or the program won't work as expected. I don't
see a possible deadlock here, though.

For example - writing to /proc/self/fd/0 is ok; the state of fd[0] of that
task will be captured at some point in the middle of the checkpoint, so
after restart one cannot assume anything about the file position; the rest
should work.

Oren.

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