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Date:   Tue, 25 Oct 2022 18:16:53 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Eric Bergen <ebergen@meta.com>
Subject: Re: [PATCH] mm: vmscan: split khugepaged stats from direct reclaim
 stats
Message-ID: <Y1gaBXp+IENVIU7z@casper.infradead.org>
References: <20221025170519.314511-1-hannes@cmpxchg.org>
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On Tue, Oct 25, 2022 at 01:05:19PM -0400, Johannes Weiner wrote:
> +static int reclaimer_offset(void)
> +{
> +	BUILD_BUG_ON(PGSTEAL_DIRECT - PGSTEAL_KSWAPD != 1);
> +	BUILD_BUG_ON(PGSTEAL_KHUGEPAGED - PGSTEAL_KSWAPD != 2);
> +	BUILD_BUG_ON(PGDEMOTE_DIRECT - PGDEMOTE_KSWAPD != 1);
> +	BUILD_BUG_ON(PGDEMOTE_KHUGEPAGED - PGDEMOTE_KSWAPD != 2);
> +	BUILD_BUG_ON(PGSCAN_DIRECT - PGSCAN_KSWAPD != 1);
> +	BUILD_BUG_ON(PGSCAN_KHUGEPAGED - PGSCAN_KSWAPD != 2);
> +
> +	if (current_is_kswapd())
> +		return 0;
> +	if (current_is_khugepaged())
> +		return 2;
> +	return 1;
> +}

Would this be simpler as ...

	BUILD_BUG_ON(PGSTEAL_DIRECT - PGSTEAL_KSWAPD !=
			PGDEMOTE_DIRECT - PGDEMOTE_KSWAPD);
	BUILD_BUG_ON(PGSTEAL_DIRECT - PGSTEAL_KSWAPD !=
			PGSCAN_DIRECT - PGSCAN_KSWAPD);
	BUILD_BUG_ON(PGSTEAL_KHUGEPAGED - PGSTEAL_KSWAPD !=
			PGDEMOTE_KHUGEPAGED - PGDEMOTE_KSWAPD);
	BUILD_BUG_ON(PGSTEAL_KHUGEPAGED - PGSTEAL_KSWAPD !=
			PGSCAN_KHUGEPAGED - PGDEMOTE_KSWAPD);

	if (current_is_kswapd())
		return 0;
	if (current_is_khugepaged())
		return PGSTEAL_KHUGEPAGED - PGSTEAL_KSWAPD;
	return PGSTEAL_DIRECT - PGSTEAL_KSWAPD;

Not that I think we'd ever want to separate them, but it is perhaps a
bit less magic?