linux-kernel - Re: [RFC][PATCH] Cross Memory Attach

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Date:	Thu, 16 Sep 2010 11:47:43 +0930
From:	Christopher Yeoh <cyeoh@....ibm.com>
To:	Valdis.Kletnieks@...edu
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] Cross Memory Attach

On Wed, 15 Sep 2010 12:07:11 -0400
Valdis.Kletnieks@...edu wrote:
> 
> Interesting, and nice benchmark results.  I have a question though:
> 
> > +	/* Get the pages we're interested in */
> > +	pages_pinned = get_user_pages(task, task->mm, pa,
> > +				      nr_pages_to_copy,
> > +				      copy_to, 0, process_pages,
> > NULL); +
> > +	if (pages_pinned != nr_pages_to_copy)
> > +		goto end;
> 
> ...
> 
> > +end:
> > +	for (i = 0; i < pages_pinned; i++) {
> > +		if (copy_to)
> > +			set_page_dirty_lock(process_pages[i]);
> > +		put_page(process_pages[i]);
> > +	}
> 
> It looks to me like if get_user_pages() fails to pin *all* the pages,
> we treat the target pages as dirty even though we never actually
> touched them?
> 
> Maybe it should be 'if (copy_to && *bytes_copied)'?

Yes that can happen, though the *bytes_copied check doesn't completely
fix it as copy_from_user could fail resulting in some pages being
touched, but not all of them.  I'll add some code to only call
set_page_dirty_lock on pages that really have been touched...

Regards,

Chris
-- 
cyeoh@...ibm.com
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