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Date: Thu, 27 Oct 2016 14:37:26 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: David Herrmann <dh.herrmann@gmail.com>
Cc: linux-kernel@vger.kernel.org, Andy Lutomirski <luto@amacapital.net>,
        Jiri Kosina <jikos@kernel.org>, Greg KH <greg@kroah.com>,
        Hannes Reinecke <hare@suse.com>, Steven Rostedt <rostedt@goodmis.org>,
        Arnd Bergmann <arnd@arndb.de>, Tom Gundersen <teg@jklm.no>,
        Josh Triplett <josh@joshtriplett.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [RFC v1 04/14] bus1: util - fixed list utility library
Message-ID: <20161027123726.GD3175@twins.programming.kicks-ass.net>
References: <20161026191810.12275-1-dh.herrmann@gmail.com>
 <20161026191810.12275-5-dh.herrmann@gmail.com>
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On Wed, Oct 26, 2016 at 09:18:00PM +0200, David Herrmann wrote:
> +		e = kmalloc_array(sizeof(*e), BUS1_FLIST_BATCH + 1, gfp);

> +#define BUS1_FLIST_BATCH (1024)

> +struct bus1_flist {
> +	union {
> +		struct bus1_flist *next;
> +		void *ptr;
> +	};
> +};

So that's an allocation of 8*(1024+1), or slightly more than 2 pages.

kmalloc will round up to the next power of two, so you'll end up with an
allocation of 16*1024, wasting a whopping 8184 bytes per such allocation
in slack space.

Please consider using 1023 or something for your batch size, 511 would
get you to exactly 1 page which would be even better.