linux-kernel - Re: [PATCH v2 1/2] mm/pageblock: mitigation cmpxchg false sharing in pageblock flags

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Date:   Sun, 30 Aug 2020 11:18:23 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Alex Shi <alex.shi@...ux.alibaba.com>
Cc:     Anshuman Khandual <anshuman.khandual@....com>,
        David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] mm/pageblock: mitigation cmpxchg false sharing in
 pageblock flags

On Sun, Aug 30, 2020 at 06:14:33PM +0800, Alex Shi wrote:
> +++ b/mm/page_alloc.c
> @@ -532,9 +536,18 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
>  	mask <<= bitidx;
>  	flags <<= bitidx;
>  
> +#ifdef	CONFIG_CPU_V6
> +	byte = (unsigned long)READ_ONCE(bitmap[byte_bitidx]);
> +#else
>  	byte = READ_ONCE(bitmap[byte_bitidx]);
> +#endif
>  	for (;;) {
> +#ifdef	CONFIG_CPU_V6
> +		/* arm v6 has no cmpxchgb function, so still false sharing long word */
> +		old_byte = cmpxchg((unsigned long*)&bitmap[byte_bitidx], byte, (byte & ~mask) | flags);
> +#else
>  		old_byte = cmpxchg(&bitmap[byte_bitidx], byte, (byte & ~mask) | flags);
> +#endif

Good grief, no.  Either come up with an appropriate abstraction or
abandon this patch.  We can't possibly put this kind of ifdef in the
memory allocator!