linux-kernel - Re: [PATCH 5/6] mm: Fold page->_last_nid into page->flags where possible

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Date:	Tue, 22 Jan 2013 14:46:59 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Simon Jeons <simon.jeons@...il.com>,
	Wanpeng Li <liwanp@...ux.vnet.ibm.com>,
	Hugh Dickins <hughd@...gle.com>, Linux-MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/6] mm: Fold page->_last_nid into page->flags where
 possible

On Tue, 22 Jan 2013 17:12:41 +0000
Mel Gorman <mgorman@...e.de> wrote:

> From: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> 
> page->_last_nid fits into page->flags on 64-bit. The unlikely 32-bit NUMA
> configuration with NUMA Balancing will still need an extra page field.
> As Peter notes "Completely dropping 32bit support for CONFIG_NUMA_BALANCING
> would simplify things, but it would also remove the warning if we grow
> enough 64bit only page-flags to push the last-cpu out."

How much space remains in the 64-bit page->flags?

Was this the best possible use of the remaining space?

It's good that we can undo this later by flipping
LAST_NID_NOT_IN_PAGE_FLAGS.

> [mgorman@...e.de: Minor modifications]
> Signed-off-by: Mel Gorman <mgorman@...e.de>

Several of these patches are missing signoffs (Peter and Hugh).

>
> ...
>
> +static inline int page_last_nid(struct page *page)
> +{
> +	return (page->flags >> LAST_NID_PGSHIFT) & LAST_NID_MASK;
> +}
> +
> +static inline int page_xchg_last_nid(struct page *page, int nid)
> +{
> +	unsigned long old_flags, flags;
> +	int last_nid;
> +
> +	do {
> +		old_flags = flags = page->flags;
> +		last_nid = page_last_nid(page);
> +
> +		flags &= ~(LAST_NID_MASK << LAST_NID_PGSHIFT);
> +		flags |= (nid & LAST_NID_MASK) << LAST_NID_PGSHIFT;
> +	} while (unlikely(cmpxchg(&page->flags, old_flags, flags) != old_flags));
> +
> +	return last_nid;
> +}
> +
> +static inline void reset_page_last_nid(struct page *page)
> +{
> +	page_xchg_last_nid(page, (1 << LAST_NID_SHIFT) - 1);
> +}

page_xchg_last_nid() and reset_page_last_nid() are getting nuttily
large.  Please investigate uninlining them?

reset_page_last_nid() is poorly named.  page_reset_last_nid() would be
better, and consistent.


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