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Date: Mon, 23 Jul 2007 16:00:24 -0700 (PDT)
From: Christoph Lameter <clameter@sgi.com>
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To: Andrew Morton <akpm@linux-foundation.org>
cc: Linus Torvalds <torvalds@linux-foundation.org>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>,
       linux-kernel <linux-kernel@vger.kernel.org>,
       Daniel Phillips <phillips@google.com>, linux-mm <linux-mm@kvack.org>
Subject: Re: [PATCH] add __GFP_ZERP to GFP_LEVEL_MASK
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On Mon, 23 Jul 2007, Andrew Morton wrote:

> So this:
> 
> 	/*
> 	 * Be lazy and only check for valid flags here,  keeping it out of the
> 	 * critical path in kmem_cache_alloc().
> 	 */
> 	BUG_ON(flags & ~(GFP_DMA | __GFP_ZERO | GFP_LEVEL_MASK));
> 
> would no longer need the __GFP_ZERO.  Ditto in slob's new_slab().

That __GFP_ZERO is needed to avoid triggering the BUG_ON. The next line

	local_flags = (flags & GFP_LEVEL_MASK);

filters out the __GFP_ZERO before calling the page allocator.