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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