Re: [PATCH 5/8] mm/slub: Factor out some common code.

From: David Rientjes
Date: Wed Mar 16 2011 - 17:51:41 EST

On Wed, 16 Mar 2011, George Spelvin wrote:

> > Where's your signed-off-by?
> Somewhere under the pile of crap on my desk. :-)
> (More to the point, waiting for me to think it's good enough to submit
> For Real.)

Patches that you would like to propose but don't think are ready for merge
should have s/PATCH/RFC/ done on the subject line.

> > Nice cleanup.
> >
> > "flag" should be unsigned long in all of these functions: the constants
> > are declared with UL suffixes in slab.h.
> Actually, I did that deliberately. Because there's a problem I keep
> wondering about, which repeats many many times in the kernel:

You deliberately created a helper function to take an unsigned int when
the actuals being passed in are all unsigned long to trigger a discussion
on why they are unsigned long?

> *Why* are they unsigned long? That's an awkward type: 32 bits on many
> architectures, so we can't portably assign more than 32 bits, and on
> platforms where it's 64 bits, the upper 32 are just wasting space.
> (And REX prefixes on x86-64.)

unsigned long uses the native word size of the architecture which can
generate more efficient code; we typically imply that flags have a limited
size by including leading zeros in their definition for 32-bit

#define SLAB_DEBUG_FREE 0x00000100UL /* DEBUG: Perform (expensive) checks on free */
#define SLAB_RED_ZONE 0x00000400UL /* DEBUG: Red zone objs in a cache */
#define SLAB_POISON 0x00000800UL /* DEBUG: Poison objects */
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