linux-kernel - Re: [RFC][PATCH 3/5] check files for checkpointability

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Message-ID: <20090224011037.GB4797@us.ibm.com>
Date:	Mon, 23 Feb 2009 19:10:37 -0600
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
Cc:	Alexey Dobriyan <adobriyan@...il.com>, Ingo Molnar <mingo@...e.hu>,
	containers <containers@...ts.linux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH 3/5] check files for checkpointability

Quoting Dave Hansen (dave@...ux.vnet.ibm.com):
> On Mon, 2009-02-23 at 17:49 -0600, Serge E. Hallyn wrote:
> > Quoting Dave Hansen (dave@...ux.vnet.ibm.com): 
> > > Introduce a files_struct counter to indicate whether a particular
> > > file_struct has ever contained a file which can not be
> > > checkpointed.  This flag is a one-way trip; once it is set, it may
> > > not be unset.
> > > 
> > > We assume at allocation that a new files_struct is clean and may
> > > be checkpointed.  However, as soon as it has had its files filled
> > > from its parent's, we check it for real in __scan_files_for_cr().
> > > At that point, we mark it if it contained any uncheckpointable
> > > files.
> > > 
> > > We also check each 'struct file' when it is installed in a fd
> > > slot.  This way, if anyone open()s or managed to dup() an
> > > unsuppored file, we can catch it.
> > 
> > So what is the point of tagging the files_struct counter and
> > making it a one-way trip?  Why not just check every file at
> > checkpoint time?
> 
> We need both.
> 
> This allows us to tell where and when we went wrong.  Take a process
> that's been running for a month.  After 5 days it did something random
> to keep it from being checkpointed.  You're going to have forgotten all
> about it 25 days later.  This gives us an opportunity to spit into dmesg
> or just plain log it.  It also gives the app some ability to reflect and
> see what its uncheckpointable attributes are.  

Hmm.  In that case, rather than refuse checkpoint, I prefer that we make
this a footnote in the /proc/$$/checkpointable output.

-serge
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