linux-kernel - Re: [PATCH 08/17] mm: page_alloc: Use word-based accesses for get/set pageblock bitmaps

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Message-ID: <20140506203449.GG1429@laptop.programming.kicks-ass.net>
Date:	Tue, 6 May 2014 22:34:49 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Linux-MM <linux-mm@...ck.org>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Vlastimil Babka <vbabka@...e.cz>, Jan Kara <jack@...e.cz>,
	Michal Hocko <mhocko@...e.cz>, Hugh Dickins <hughd@...gle.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 08/17] mm: page_alloc: Use word-based accesses for
 get/set pageblock bitmaps

On Thu, May 01, 2014 at 09:44:39AM +0100, Mel Gorman wrote:
> +void set_pfnblock_flags_group(struct page *page, unsigned long flags,
> +					unsigned long end_bitidx,
> +					unsigned long nr_flag_bits,
> +					unsigned long mask)
>  {
>  	struct zone *zone;
>  	unsigned long *bitmap;
> +	unsigned long pfn, bitidx, word_bitidx;
> +	unsigned long old_word, new_word;
> +
> +	BUILD_BUG_ON(NR_PAGEBLOCK_BITS != 4);
>  
>  	zone = page_zone(page);
>  	pfn = page_to_pfn(page);
>  	bitmap = get_pageblock_bitmap(zone, pfn);
>  	bitidx = pfn_to_bitidx(zone, pfn);
> +	word_bitidx = bitidx / BITS_PER_LONG;
> +	bitidx &= (BITS_PER_LONG-1);
> +
>  	VM_BUG_ON_PAGE(!zone_spans_pfn(zone, pfn), page);
>  
> +	bitidx += end_bitidx;
> +	mask <<= (BITS_PER_LONG - bitidx - 1);
> +	flags <<= (BITS_PER_LONG - bitidx - 1);
> +
> +	do {
> +		old_word = ACCESS_ONCE(bitmap[word_bitidx]);
> +		new_word = (old_word & ~mask) | flags;
> +	} while (cmpxchg(&bitmap[word_bitidx], old_word, new_word) != old_word);
>  }

You could write it like:

	word = ACCESS_ONCE(bitmap[word_bitidx]);
	for (;;) {
		old_word = cmpxchg(&bitmap[word_bitidx], word, (word & ~mask) | flags);
		if (word == old_word);
			break;
		word = old_word;
	}

It has a slightly tighter loop by avoiding the read being included.
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