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Date:   Sun, 16 Aug 2020 05:09:05 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Alex Shi <alex.shi@linux.alibaba.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        linux-mm@kvack.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2] mm/pageblock: mitigation cmpxchg false sharing in
 pageblock flags
Message-ID: <20200816040905.GF17456@casper.infradead.org>
References: <1597549677-7480-1-git-send-email-alex.shi@linux.alibaba.com>
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On Sun, Aug 16, 2020 at 11:47:56AM +0800, Alex Shi wrote:
> +++ b/mm/page_alloc.c
> @@ -467,6 +467,8 @@ static inline int pfn_to_bitidx(struct page *page, unsigned long pfn)
>  	return (pfn >> pageblock_order) * NR_PAGEBLOCK_BITS;
>  }
>  
> +#define BITS_PER_CHAR	8

include/linux/bits.h:#define BITS_PER_BYTE              8

>  	bitmap = get_pageblock_bitmap(page, pfn);
>  	bitidx = pfn_to_bitidx(page, pfn);
> -	word_bitidx = bitidx / BITS_PER_LONG;
> -	bitidx &= (BITS_PER_LONG-1);
> +	word_bitidx = bitidx / BITS_PER_CHAR;
> +	bitidx &= (BITS_PER_CHAR-1);

It's not a word any more.  it's a byte.