linux-kernel - Re: [RFC PATCH] <linux/taggedptr.h>: Introduce tagged pointer

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Message-ID: <20180628092303.GD7646@bombadil.infradead.org>
Date:   Thu, 28 Jun 2018 02:23:03 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Gao Xiang <gaoxiang25@...wei.com>
Cc:     viro@...iv.linux.org.uk,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Chao Yu <yuchao0@...wei.com>, Miao Xie <miaoxie@...wei.com>,
        linux-fsdevel@...r.kernel.org, linux-erofs@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] <linux/taggedptr.h>: Introduce tagged pointer

On Thu, Jun 28, 2018 at 05:06:29PM +0800, Gao Xiang wrote:
> Therefore, this patch introduces simple generic methods to fold
> tags into a pointer integer. It currently reuses the last 2 bits
> of the pointer for tags, which are safely for all modern platforms.

The m68k people will have your head.  alignof(unsigned long) == 2 on
m68k.  Now, kmalloc always returns 8-byte aligned quantities, but
if you have:

static unsigned long foo;

then ((unsigned long)&foo & 2) may be non-zero.

> +/*
> + * mark these special integers as another type
> + * in order to highlight the tagged pointer usage.
> + */
> +typedef uintptr_t	taggedptr_t;

I find this a bit verbose.  How about tagptr_t ?