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Date: Wed, 15 Apr 2015 13:42:20 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Mel Gorman <mgorman@suse.de>
Cc: Linux-MM <linux-mm@kvack.org>, Rik van Riel <riel@redhat.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Dave Hansen <dave.hansen@intel.com>, Andi Kleen <andi@firstfloor.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 3/4] mm: Gather more PFNs before sending a TLB to flush
 unmapped pages
Message-ID: <20150415114220.GG17717@twins.programming.kicks-ass.net>
References: <1429094576-5877-1-git-send-email-mgorman@suse.de>
 <1429094576-5877-4-git-send-email-mgorman@suse.de>
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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.