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

[Vlastimil Babka]

On 05/06/2014 11:13 AM, Mel Gorman wrote:
> 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.
> > fix:
>
> 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.

I was wondering about hardware guarantees in that case (e.g. consistency 
at least on the granularity of byte when a simple memory read races with 
write) but after some discussion in the office I understand that 
hardware without such guarantees wouldn't be able to run Linux anyway :)

Still I wonder if ACCESS_ONCE would be safer in the 'word' variable 
assignment to protect against compiler trying to be too smart?

Anyway with the nr_flag_bits removed:

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> > > +	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.

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