From: Matthew Wilcox <willy@infradead.org>
To: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
linux-fsdevel@vger.kernel.org
Cc: Gao Xiang <gaoxiang25@huawei.com>,
zhong jiang <zhongjiang@huawei.com>, Chao Yu <yuchao0@huawei.com>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Subject: Re: Tagged pointers in the XArray
Date: Wed, 29 Aug 2018 09:17:56 -0700 [thread overview]
Message-ID: <20180829161756.GB30396@bombadil.infradead.org> (raw)
In-Reply-To: <20180828222727.GD11400@bombadil.infradead.org>
On Tue, Aug 28, 2018 at 03:27:27PM -0700, Matthew Wilcox wrote:
> I find myself caught between two traditions.
>
> On the one hand, the radix tree has been calling the page cache dirty &
> writeback bits "tags" for over a decade.
>
> On the other hand, using some of the bits _in a pointer_ as a tag has been
> common practice since at least the 1960s.
> https://en.wikipedia.org/wiki/Tagged_pointer and
> https://en.wikipedia.org/wiki/31-bit
>
> EROFS wants to use tagged pointers in the radix tree / xarray. Right now,
> they're building them by hand, which is predictably grotty-looking.
> I think it's reasonable to provide this functionality as part of the
> XArray API, _but_ it's confusing to have two different things called tags.
>
> I've done my best to document my way around this, but if we want to rename
> the things that the radix tree called tags to avoid the problem entirely,
> now is the time to do it. Anybody got a Good Idea?
I have two ideas now.
First, we could rename radix tree tags to xarray marks. That is,
xa_mark_t
xa_set_mark()
xas_clear_mark()
xas_for_each_marked() { }
xa_marked()
etc
Second, we could call the tagged pointers typed pointers. That is,
void *xa_mk_type(void *p, unsigned int type);
void *xa_to_ptr(void *entry);
int xa_ptr_type(void *entry);
Any better ideas, or violent revulsion to either of the above ideas?
prev parent reply other threads:[~2018-08-29 16:17 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-08-28 22:27 Tagged pointers in the XArray Matthew Wilcox
2018-08-28 22:39 ` Randy Dunlap
2018-08-28 23:03 ` Matthew Wilcox
2018-08-28 23:09 ` Randy Dunlap
2018-08-28 23:24 ` Gao Xiang
2018-08-28 23:26 ` Gao Xiang
2018-08-29 16:17 ` Matthew Wilcox [this message]
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