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SubjectRe: [PATCH] add __GFP_ZERO to GFP_LEVEL_MASK
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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@chello.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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