linux-kernel - Re: [RFC][PATCH 8/8] check files for checkpointability

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Date:	Wed, 04 Mar 2009 15:41:41 -0800
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	containers <containers@...ts.linux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Serge E. Hallyn" <serue@...ibm.com>,
	Oren Laadan <orenl@...columbia.edu>,
	Alexey Dobriyan <adobriyan@...il.com>, hch@...radead.org
Subject: Re: [RFC][PATCH 8/8] check files for checkpointability

On Fri, 2009-02-27 at 18:57 -0800, Sukadev Bhattiprolu wrote: 
> Dave Hansen [dave@...ux.vnet.ibm.com] wrote:
> | 
> | 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.
> 
> Hmm. Why not just copy ->may_checkpoint setting from parent (or old)
> files_struct ? If parent is not checkpointable, then child won't be
> and vice-versa - no ?

Because init does things that are uncheckpointable.  If we purely
inherit, we'll never be able to checkpoint.

> | +static void __scan_files_for_cr(struct files_struct *files)
> | +{
> | +	int i;
> | +
> | +	for (i = 0; i < files->fdtab.max_fds; i++) {
> | +		struct file *f = fcheck_files(files, i);
> | +		if (!f)
> | +			continue;
> | +		if (cr_file_supported(f))
> | +			continue;
> | +		files_deny_checkpointing(files);
> | +	}
> | +}
> | +
> 
> A version of __scan_files_for_cr() for CONFIG_CHECKPOINT_RESTART=n or...
> 
> |  /*
> |   * Allocate a new files structure and copy contents from the
> |   * passed in files structure.
> | @@ -303,6 +318,9 @@ struct files_struct *dup_fd(struct files
> |  		goto out;
> | 
> |  	atomic_set(&newf->count, 1);
> | +#ifdef CONFIG_CHECKPOINT_RESTART
> | +	newf->may_checkpoint = 1;
> | +#endif
> | 
> |  	spin_lock_init(&newf->file_lock);
> |  	newf->next_fd = 0;
> | @@ -396,6 +414,7 @@ struct files_struct *dup_fd(struct files
> | 
> |  	rcu_assign_pointer(newf->fdt, new_fdt);
> | 
> | +	__scan_files_for_cr(newf);
> 
> ... #ifdef CONFIG_CHECKPOINT_RESTART here ?

gcc isn't quite smart enough to figure out that this is a noop.  Please
see my new set for a new compiler helper to solve this problem.

-- Dave

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