linux-kernel - Re: [PATCH] add __GFP_ZERO to GFP_LEVEL

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Message-Id: <1185256869.8197.27.camel@twins>
Date:	Tue, 24 Jul 2007 08:01:09 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Christoph Lameter <clameter@....com>
Cc:	Mel Gorman <mel@...net.ie>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Daniel Phillips <phillips@...gle.com>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH] add __GFP_ZERO to GFP_LEVEL_MASK

On Mon, 2007-07-23 at 16:17 -0700, Christoph Lameter wrote:
> On Mon, 23 Jul 2007, Peter Zijlstra wrote:
> 
> > ---
> > Daniel recently spotted that __GFP_ZERO is not (and has never been)
> > part of GFP_LEVEL_MASK. I could not find a reason for this in the
> > original patch: 3977971c7f09ce08ed1b8d7a67b2098eb732e4cd in the -bk
> > tree.
> > 
> > This of course is in stark contradiction with the comment accompanying
> > GFP_LEVEL_MASK.
> 
> NACK.
> 
> The effect that this patch will have is that __GFP_ZERO is passed through 
> to the page allocator which will needlessly zero pages. GFP_LEVEL_MASK is 
> used to filter out the flags that are to be passed to the page allocator. 
> __GFP_ZERO is not passed on but handled by the slab allocators.

Then we can either fixup the slab allocators to mask out __GFP_ZERO, or
do something like the below.

Personally I like the consistency of adding __GFP_ZERO here (removes
this odd exception) and just masking it in the sl[aou]b thingies.

Anybody else got a preference?

Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
---
 include/linux/gfp.h |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

Index: linux-2.6-2/include/linux/gfp.h
===================================================================
--- linux-2.6-2.orig/include/linux/gfp.h
+++ linux-2.6-2/include/linux/gfp.h
@@ -53,7 +53,14 @@ struct vm_area_struct;
 #define __GFP_BITS_SHIFT 20	/* Room for 20 __GFP_FOO bits */
 #define __GFP_BITS_MASK ((__force gfp_t)((1 << __GFP_BITS_SHIFT) - 1))
 
-/* if you forget to add the bitmask here kernel will crash, period */
+/*
+ * If you forget to add the bitmask here kernel will crash, period!
+ *
+ * GFP_LEVEL_MASK is used to filter out the flags that are to be passed to the
+ * page allocator.
+ *
+ * __GFP_ZERO is not passed on but handled by the slab allocators.
+ */
 #define GFP_LEVEL_MASK (__GFP_WAIT|__GFP_HIGH|__GFP_IO|__GFP_FS| \
 			__GFP_COLD|__GFP_NOWARN|__GFP_REPEAT| \
 			__GFP_NOFAIL|__GFP_NORETRY|__GFP_COMP| \


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