From: Peter Seebach <peter.seebach@windriver.com>
To: Michal Nazarewicz <mina86@mina86.com>
Cc: Christoph Lameter <cl@linux-foundation.org>,
Pekka Enberg <penberg@kernel.org>, Matt Mackall <mpm@selenic.com>,
Tejun Heo <tj@kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
Lai Jiangshan <laijs@cn.fujitsu.com>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [RFC PATCH 1/6] kenrel.h: add ALIGN_OF_LAST_BIT()
Date: Tue, 20 Mar 2012 09:20:58 -0500 [thread overview]
Message-ID: <20120320092058.1b3271a4@wrlaptop> (raw)
In-Reply-To: <op.wbgvn00x3l0zgt@mpn-glaptop>
On Tue, 20 Mar 2012 12:32:14 +0100
Michal Nazarewicz <mina86@mina86.com> wrote:
> >+#define ALIGN_OF_LAST_BIT(x) ((((x)^((x) - 1))>>1) + 1)
>
> Wouldn't ALIGNMENT() be less confusing? After all, that's what this
> macro is calculating, right? Alignment of given address.
Why not just LAST_BIT(x)? It's not particularly specific to pointer
alignment, even though that's the context in which it apparently came
up. So far as I can tell, this isn't even meaningfully defined on
pointer types as such; you'd have to convert. So the implications for
alignment seem a convenient side-effect, really.
It might be instructive to see some example proposed uses; the question
of why I'd care what alignment something had, rather than whether it
was aligned for a given type, is one that will doubtless keep me awake
nights.
I guess this feels like it answers a question that is usually the wrong
question. Imagine if you will a couple-page block of memory, full of
unsigned shorts. Iterate through the array, calculating
ALIGN_OF_LAST_BIT(&a[i]). Do we really *care* that it's PAGE_SIZE for
some i, and 2 (I assume) for other i, and PAGE_SIZE*2 for either i==0 or
i==PAGE_SIZE? (Apologies if this is a silly question; maybe this is
such a commonly-needed feature that it's obvious.)
-s
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next prev parent reply other threads:[~2012-03-20 14:22 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-03-20 10:21 [RFC PATCH 0/6] add kmalloc_align() Lai Jiangshan
2012-03-20 10:21 ` [RFC PATCH 1/6] kenrel.h: add ALIGN_OF_LAST_BIT() Lai Jiangshan
2012-03-20 11:32 ` Michal Nazarewicz
2012-03-20 14:03 ` Alexey Dobriyan
2012-03-20 14:08 ` Christoph Lameter
2012-03-20 14:20 ` Peter Seebach [this message]
2012-03-20 10:21 ` [RFC PATCH 2/6] slub: add kmalloc_align() Lai Jiangshan
2012-03-20 14:14 ` Christoph Lameter
2012-03-20 14:21 ` Christoph Lameter
2012-03-20 10:21 ` [RFC PATCH 3/6] slab: " Lai Jiangshan
2012-03-20 10:21 ` [RFC PATCH 4/6] slob: don't couple the header size with the alignment Lai Jiangshan
2012-03-20 10:21 ` [RFC PATCH 5/6] slob: add kmalloc_align() Lai Jiangshan
2012-03-20 10:21 ` [RFC PATCH 6/6] workqueue: use kmalloc_align() instead of hacking Lai Jiangshan
2012-03-20 15:15 ` Christoph Lameter
2012-03-20 15:46 ` Tejun Heo
2012-03-21 3:02 ` Lai Jiangshan
2012-03-21 5:14 ` Tejun Heo
2012-03-21 14:12 ` Patch workqueue: create new slab cache " Christoph Lameter
2012-03-21 14:49 ` Eric Dumazet
2012-03-21 15:03 ` Christoph Lameter
2012-03-21 16:04 ` Eric Dumazet
2012-03-21 17:54 ` Christoph Lameter
2012-03-21 18:05 ` Eric Dumazet
2012-03-21 18:20 ` Christoph Lameter
2012-03-21 16:09 ` Tejun Heo
2012-03-21 17:56 ` Christoph Lameter
2012-03-21 13:45 ` [RFC PATCH 6/6] workqueue: use kmalloc_align() " Christoph Lameter
2012-03-26 2:00 ` Lai Jiangshan
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