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

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Date:	Wed, 15 Sep 2010 12:07:11 -0400
From:	Valdis.Kletnieks@...edu
To:	Christopher Yeoh <cyeoh@....ibm.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH] Cross Memory Attach

On Wed, 15 Sep 2010 10:48:55 +0930, Christopher Yeoh said:

> The basic idea behind cross memory attach is to allow MPI programs doing
> intra-node communication to do a single copy of the message rather than
> a double copy of the message via shared memory.

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)'?

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