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

[Matthew Wilcox]

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!