linux-kernel - Re: [PATCH 3/4] mm: Gather more PFNs before sending a TLB to flush unmapped pages

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Date:	Wed, 15 Apr 2015 13:42:20 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Linux-MM <linux-mm@...ck.org>, Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Dave Hansen <dave.hansen@...el.com>,
	Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] mm: Gather more PFNs before sending a TLB to flush
 unmapped pages

On Wed, Apr 15, 2015 at 11:42:55AM +0100, Mel Gorman wrote:
> +/*
> + * Use a page to store as many PFNs as possible for batch unmapping. Adjusting
> + * this trades memory usage for number of IPIs sent
> + */
> +#define BATCH_TLBFLUSH_SIZE \
> +	((PAGE_SIZE - sizeof(struct cpumask) - sizeof(unsigned long)) / sizeof(unsigned long))
>  
>  /* Track pages that require TLB flushes */
>  struct unmap_batch {
> +	/* Update BATCH_TLBFLUSH_SIZE when adjusting this structure */
>  	struct cpumask cpumask;
>  	unsigned long nr_pages;
>  	unsigned long pfns[BATCH_TLBFLUSH_SIZE];

The alternative is something like:

struct unmap_batch {
	struct cpumask cpumask;
	unsigned long nr_pages;
	unsigned long pfnsp[0];
};

#define BATCH_TLBFLUSH_SIZE ((PAGE_SIZE - sizeof(struct unmap_batch)) / sizeof(unsigned long))

and unconditionally allocate 1 page. This saves you from having to worry
about the layout of struct unmap_batch.
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