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

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Date:	Tue, 06 May 2014 16:42:18 +0200
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Mel Gorman <mgorman@...e.de>
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 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@...e.cz>

>>> +	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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