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

[Peter Zijlstra <peterz@infradead.org>]

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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