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

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Date:	Tue, 6 May 2014 10:13:36 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Vlastimil Babka <vbabka@...e.cz>
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	Linux-MM <linux-mm@...ck.org>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.org>,
	Johannes Weiner <hannes@...xchg.org>, 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 Mon, May 05, 2014 at 02:40:38PM +0200, Vlastimil Babka wrote:
> >@@ -62,11 +65,35 @@ extern int pageblock_order;
> >  /* Forward declaration */
> >  struct page;
> >
> >+unsigned long get_pageblock_flags_mask(struct page *page,
> >+				unsigned long end_bitidx,
> >+				unsigned long nr_flag_bits,
> >+				unsigned long mask);
> >+void set_pageblock_flags_mask(struct page *page,
> >+				unsigned long flags,
> >+				unsigned long end_bitidx,
> >+				unsigned long nr_flag_bits,
> >+				unsigned long mask);
> >+
> 
> The nr_flag_bits parameter is not used anymore and can be dropped.
> 

Fixed

> >  /* Declarations for getting and setting flags. See mm/page_alloc.c */
> >-unsigned long get_pageblock_flags_group(struct page *page,
> >-					int start_bitidx, int end_bitidx);
> >-void set_pageblock_flags_group(struct page *page, unsigned long flags,
> >-					int start_bitidx, int end_bitidx);
> >+static inline unsigned long get_pageblock_flags_group(struct page *page,
> >+					int start_bitidx, int end_bitidx)
> >+{
> >+	unsigned long nr_flag_bits = end_bitidx - start_bitidx + 1;
> >+	unsigned long mask = (1 << nr_flag_bits) - 1;
> >+
> >+	return get_pageblock_flags_mask(page, end_bitidx, nr_flag_bits, mask);
> >+}
> >+
> >+static inline void set_pageblock_flags_group(struct page *page,
> >+					unsigned long flags,
> >+					int start_bitidx, int end_bitidx)
> >+{
> >+	unsigned long nr_flag_bits = end_bitidx - start_bitidx + 1;
> >+	unsigned long mask = (1 << nr_flag_bits) - 1;
> >+
> >+	set_pageblock_flags_mask(page, flags, end_bitidx, nr_flag_bits, mask);
> >+}
> >
> >  #ifdef CONFIG_COMPACTION
> >  #define get_pageblock_skip(page) \
> >diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> >index dc123ff..f393b0e 100644
> >--- a/mm/page_alloc.c
> >+++ b/mm/page_alloc.c
> >@@ -6032,53 +6032,64 @@ static inline int pfn_to_bitidx(struct zone *zone, unsigned long pfn)
> >   * @end_bitidx: The last bit of interest
> >   * returns pageblock_bits flags
> >   */
> >-unsigned long get_pageblock_flags_group(struct page *page,
> >-					int start_bitidx, int end_bitidx)
> >+unsigned long get_pageblock_flags_mask(struct page *page,
> >+					unsigned long end_bitidx,
> >+					unsigned long nr_flag_bits,
> >+					unsigned long mask)
> >  {
> >  	struct zone *zone;
> >  	unsigned long *bitmap;
> >-	unsigned long pfn, bitidx;
> >-	unsigned long flags = 0;
> >-	unsigned long value = 1;
> >+	unsigned long pfn, bitidx, word_bitidx;
> >+	unsigned long word;
> >
> >  	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);
> >
> >-	for (; start_bitidx <= end_bitidx; start_bitidx++, value <<= 1)
> >-		if (test_bit(bitidx + start_bitidx, bitmap))
> >-			flags |= value;
> >-
> >-	return flags;
> >+	word = bitmap[word_bitidx];
> 
> I wonder if on some architecture this may result in inconsistent
> word when racing with set(), i.e. cmpxchg? We need consistency at
> least on the granularity of byte to prevent the problem with bogus
> migratetype values being read.
> 

The number of bits align on the byte boundary so I do not think there is
a problem there. There is a BUILD_BUG_ON check in set_pageblock_flags_mask
in case this changes so it can be revisited if necessary.

> >+	bitidx += end_bitidx;
> >+	return (word >> (BITS_PER_LONG - bitidx - 1)) & mask;
> 
> Yes that looks correct to me, bits don't seem to overlap anymore.
> 

Thanks.

-- 
Mel Gorman
SUSE Labs
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